In fact, make changes in common to let the player understand the slider component.

Since the options of the DropListComponent are now managed as values,
the DropListComponent can be treated as a standard FormElementComponent

Although the DropListElement is a FormElement, its value is represented
by the field options. Therefore, this state must be saved.

Improve detection of valid pages, sections and elements

It needs row and column to work. (It is using flex in the background.)

Use 'horizontal' and 'vertical'. Which are better terms and also align 
with other element's values.

Also improve conditional border style. The additional dragPreview is 
needed so the dragged item does not have the border.

The width/height of 90% are a preliminary workaround to not produce 
overflow with the border. A better solution would be good.

In the UI. Internally it is still likert.

The player-element dialog options did not show the field, even when it 
has a value. Now it properly checks of the existing config has this set.

This element has a list of string as editable options. Those strings 
form draggable elements which may be reordered or moved between 
different DropList elements.

Also refactor option change logic to allow any property not just the 
text property.

This includes a fix in the dispacker, which no more tries (and fails) to 
replace variable names.