qml: refactor ControlBar and MiniPlayer

• We can not use Control as is, just like other controls because a custom style is not applied.
• Inherit Pane template in ControlBar also because it needs to be a focus scope.
• MiniPlayer should inherit from ControlBar. Aggregation here does not make sense, they are both controls.
• Use declarative approach for layouting in MiniPlayer.
• Some other stuff, see the commits.