Allow multi item drag and resize in composer (fix #7918)

Moves responsibility for drawing selection mouse handles and mouse interaction with selection to new class "QgsComposerMouseHandles". This new class is responsible for drawing the selection mouse handles, and dotted borders around selected items. It also handles most mouse interactions with selections.

Benefits:
- Multi item drag & resize
- Selection handles aren't influenced by item opacity or blend mode,
- Handles are always drawn on top - they no longer hide below items higher in the scene