qml: player toolbar focus handling improvements

Thanks to the recent introduction of attached Navigation, this MR became much simpler. Fortunately, there is no need to use Control as a parent to toolbar widgets.

There are many edge cases that can cause the focus to get lost while navigating in the player toolbar. Including, but not limited:

• Using toolbar editor (editing the toolbar). (this even caused segmentation faults)
• Control getting disabled while being focused.
• Alignments having no control / all controls are disabled.
• Alignment getting invisible due to insufficient width.
• Control getting removed while being focused.

This MR resolves all these issues about focus handling and navigation. The only remaining issue is when all controls are removed or when all controls are disabled the focus is lost.

Fixes #25762 (closed), #25748 (closed).