Also add separate file to handle drop-logic.

#671

#664

The background color is set via data-attribute so it can be turned off 
for the parent div but still used in the child (button).

- Remove superfluous width and height. This is already done by parent 
elements in player and editor.
- Remove superfluous div and use host element instead.

Stretch across the whole line.

This fixes Tables which houses elements without dimensions.

This way the props do not show in the prop-panel.

#662

Images were not showing at all.

The default value overwrites the value set in the parent constructor 
(Javascript...). Therefore the value needs to be intialized explicitly 
after the parent constructor has run.

The index was one off, when looking up the correct child element.

Only background color makes sense here.

Remove button is permanently visible and bith buttons are now in the 
lower left corner

#643

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.

#657

as this causes images to be displayed too small on iPads
when they are first loaded.

Mouuseenter, mouseleave for list and items are now always active and no 
longer added after the drag has started (they check of a drag operation 
is going on though). This makes the code simpler and more managable.