Use a custom model for relation editor form widget

[wonder-sk]

The relation editor form widget shows a couple of related features, with a button to show a full list ("N more") if not all related features can be shown. The current implementation uses Flow+Repeater and some logic to hide extra items (and count number of hidden items) if there are too many items. This is however a bit complicated, and it may be slow when there are many related features, because Repeater will create an item for each feature even if it is hidden. The idea is to move all this complexity to a custom item model that would be also responsible for layout decisions (it will need some basic visual parameters from QML - e.g. font metrics, number of rows, spacing). Such model should be then faster, easier to auto-test and QML code would be without much logic.

[tomasMizera]

++ we might consider if something like this should be used for the gallery too - I understand that ListView is smarter in instantiating delegates, but we will want to show only the first N features and then show x more too :)