Row first.

Frames are meant to be in the background and putting elements on top is 
common with them.

This is only used by the Editor. It was in common while the 
SanitizationService was used by the Player which then could display 
errors. Sanitization has been refactored and this is no longer needed.

Separating code changes is rather hard right now, so there is a lot of stuff that is part of this commit that should not. Details below.

- All elements have static fields that holds their title and icon. This way dynamic lists can be built (for example for delete dialog) which show title and icon for all elements. ALso icons and names can be changed in one place.
- Template-Tab has been added. This should have templates to help creating groups of ui elements. This is work in progress and there are actually no templates yet. So this will change or even be removed in the future. This is mostly UI, the code is "TODO".
- Refactoring and improvement for delete and reference dialogs
- Delete logic refactored for elements, sections and pages. There are no longer different dialogs when references are found, but instead one dialog that can do both.

This sets them up from the central UnitView.

Only works for a few commands (mainly page and section stuff).

- Adds new menu to page-bar, which shows issued change commands with a 
button to rollback the latest change.
- Added warning to the UI element that this is a experimental feature 
and can destroy units.
- Undo is done by having the normal function to apply the change to the 
unit-object and another to undo this change. The HistoryService manages 
the list and calling rollack (via UnitService).
- Further refactiring to the unit services.

...and make tooltip config global.

- Encapsule into separate component
- No more pop-in-controls
- slicker ui layout
- More tooltip with explanations

#523
- Improve sanitizing of position margins

#511

In Player-context the constructors are to fail, when the given 
properties are not exactly as expected. Therefoore they are validated. 
When this validation fails, an environment variable is checked to 
determine if the element should be created anyway with defaults. This 
switch is off in player-context. But the Editor is not so strict and can 
create elements with incomplete parameters.

There is also a new VersionManager which checks given unit definitions 
for version-validity. Upon failing this check Player and Editor show 
error messages.
The Editor accepts a certain unit definition version, but shows a 
warning.

The Editor checks units for invalid references after loading. If any are 
found, they are removed and an info-panel is shown.

This hack allows the page tabs to have a pseudo tab to create a new 
page.
This tab is disabled so the user can not change to it, but is still 
clickable.

- Add ReferenceManager to check if deleted elements are referenced 
anywhere in the unit. If so display a warning.
- Move page logic to UnitService

Remove 'any's be adding proper types and using 'never'.

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

Show accordion with expansion-panels instead of tabs.
This way elements can be grouped logically.

Components with bigger scope are put to the component root, canvas, 
props panle and new-element-panel.

- Move value input to dropList input group

With this setting elements are copied when being dropped to another 
list.

On drop it is also checked if the item-ID is already present in the 
list. If it is the drop event is silently discarded. This allows putting 
items back in the list without creating duplicate IDs.

Lists with this setting:
- do show a placeholder of the items being dragged. This way it is 
conveyed that the item will remain there after being dropped.
- don't show a placeholder when items are hovered over them, to avoid 
confusion with duplicate items.

Sorting within DropLists is disabled, since it causes moving items 
within the list. It has to be discussed if this feature is even needed.

- Split text field and drop list, so they can have differing props
- Improve their dimension default values
- Improve alignment of all cloze children components
- Add dynamic width parameter of toggle buttons to dimension properties
panel. Therefore the postion tab can always be shown and no longer needs
a condition.

Remove unused class.

- Add fieldset to make it look nicer.
- Improve image to have fixed size, scale properly and show buttons only 
on hover.
- Improve translations
- Add tooltip to actionparam dropdown: When having selected pageNav it 
shows a tooltip to get across that only valid pages can show up.

#240

since the player uses it as well