For Droplists this needs to be done when multiple items are allowed.

For ToggleButtons it needs to be set when vertical orientation is used.

It is then centered without caring about text alignment. With multiple 
item it would not be possible to align all of them anyway.

- Add left and right padding for input field

- When the height of the child elements of a cloze element is increased,
they all now grow out of their center

- Extra span element around cloze child elements served no visible 
purpose. It also makes sense that an inline div is not inside of a span, 
which is meant for text.
- CSS rules were overengineered and in part shadowed (and thereby 
pointless) by element styles from the template.
This is also in preparation for the DropList styling overhaul.

- If this property is set, the input of more characters than specified
in maxLength is prevented
- It also prevents the insertion of text.

- Reduce redundant code
- Prepare implementation of input limitations

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- overflow of lists is now hidden to prevent dropable elements from
being displayed outside the container
- placeholders are hidden with flex orientation

Create new pipe for checking if element is included in list.

- Make position props on buttons optional
- Move UIElement creation back to Factory instead of within Section

Use a horizontal aligned placeholder for dynamic aligned drop lists
when there is more than one element in the list

- Refactor variable of the placeholder dimensions

- Important for vertically positioned drop lists
- Analogous to the calculation of the height

- Replace pixel calculation with percentage values
- Use of only two modes: full fill or line

#343

i.e. something has been clicked.
Since the value is not used on the player, we also need to check the 
FormControl value.

#335

- This makes the behavior identical to real form elements
- To trigger the focusout event, the HTML element needs
the tabindex attribute

simple dropdown lists need a minimum height so they don't collapse
if they don't contain any element

Using disabled cause conflicts with reactive form in player

- Add "isOlderThan" method to the "SanitizationService" to distinguish
old ToggleButtons with the property "option" from new ones
- Use SanitizationService.isUnitDefinitionOutdated in Player to set
the unitDefinitionVersion (should be modified!)

Use the same settings as in simple drop list

ViewEncapsulation.None is required for overriding tooltip

DragNDropValueObject no longer has the stringValue property
but a TextLabel with text

One as a general label above the table and one for the first column 
heading.

Since this is no proper InputElement separate properties panel inputs 
are used.

#272

To be the same as with Material inputs.

#319