No user visible changes expected.
Merges the tree row and grid tile button types, which were mostly doing
the same things. The idea is that there is a button type for
highlighting, as well as supporting general view item features (e.g.
renaming, drag/drop, etc.). So instead there is a view item button type
now. Also ports view item features like renaming, custom context menus,
drag controllers and drop controllers to `ui::AbstractViewItem` (the new
base class for all view items).
This should be quite an improvement because:
- Merges code that was duplicated over view items.
- Mentioned features (renaming, drag & drop, ...) are much easier to
implement in new view types now. Most of it comes "for free".
- Further features will immediately become availalbe to all views (e.g.
selection).
- Simplifies APIs, there don't have to be functions for individual view
item types anymore.
- View item classes are split and thus less overwhelming visually.
- View item buttons now share all code (drawing, handling, etc.)
- We're soon running out of available button types, this commit merges
two into one.
I was hoping I could do this in multiple smaller commits, but things
were quite intertwined so that would've taken quite some effort.
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