To make it work that the section itself can handle drop events, but
also have the canvas to handle drops when outside of the section, all
the allowed dropLists have to be connected. Because the lists are not
properly nested (see below), this needs to be done manually by IDs.
This list is given to the necessary dropLists to make it possible to
drop items not only into them but also any other connected dropLists.

Dynamic sections have droplists for the grid cells next to the actual
elements. Elements can not be children of the grid cells because they
can span over multiple cells.

Dynamic sections don't have a general drop area, like static sections.
They have grid placeholder elements which are droplists. Therefore they
have no parent dropList to add to the list but themselves.
Static elements only have the parent, which is added to the list.

Resizing in dynamic sections is handled by the section/element-overlays
themselves.

Ticket #38

The button is hidden when there is only one section. There always has to 
be at least one section.

Ticket: #35

The urls must be declared as trusted to be accepted by the application.

The latest change to page width and ratio needs to be reflected by a 
version change.

When this is set the page will only be rendered to that width. Otherwise 
100% of the available space is used.

This property should be used when an always visible page is used and 
states the ration of the always visible page to the other pages.

This is made for any warn message properties, which only appear when the 
corresponding velidator is actually used.

NgModel is needed so the value returned frim the field in number instead
of string.

Fix bug

* in eager mode a vopStateChangedNotification is still sent
* otherwise the last state is cached in the `VeronaPostService` and
can be retrieved from the host

Use global style for droplist cursor.

In this mode sections can be inserted, reordered and deleted with 
buttons directly on the canvas.

When form changes the `responseProgress` is determined and sent

Intersection detection is done by `Intersectionobserver`

In the past, the page index could be set by button click and after
scrolling, which led to unwanted side effects

To center the content of the player, the dimensions of all pages are
calculated.