New implementation for the KDevelop "Ideal" UI based purely on QDockWidget's:
- our custom main window and layout (IdealMainWidget and IdealMainLayout) are gone
- toolviews are now managed as regular dockwidgets (with all benefits that Qt dockwidgets bring)
- buttonbars are implemented as toolbars (not movable/closeable)

New features comparing to the old implementation:
- drag-n-drop support for toolviews (with proper toolview button positioning)
- no bugs in toolview resizing (because we no longer have our custom layout)

This change will allow us in the future to show several toolviews on one
side without significant effort.

Removed features:
- "top" area is neither allowed nor used for toolviews nor is present in UI configuration
- no "maximizing" for toolviews (this might be brought back, but only if enough people use it)
- no "anchoring"/"locking" for toolviews (this will never be brought back,
it requires custom layout which we will not try to implement anymore)