ExpandableListitem: Implement the expanded button view in a better way
The old implementation of the expanded button view used ListViews full of custom content items, with various tricky sizing hacks. This was plagued by bugs.
This commit introduces a new implementation that uses standard ToolButtons populated by a Repeater. The result is much better:
- Use of standard Plasma components, increasing consistency
- Full keyboard navigation between all items now works
- Scrolling when there is a lot of content now works properly with no stutters because the expanded views no longer have internal Flickables
- Removed a dependency on PlasmaComponents2
- Less code; fewer hidden bugs and less maintenance
There are practically no visual changes--just incidental things like elements moving a few pixels from where they were before due to using ToolButtons instead of custom list items.
Requires !329 (merged) or else the buttons look weird.