Added toolbar with icons and drag/drop support. Also refactored actor creation.

This merge request does:

• implement a Toolbar to quickly create new actors. Each actor is represented by an icon.
• implement drag/drop functionality within Met3D with supports simple addition of more drag/drop types in the future. Specifically, actors can be dragged from the UI or the OS file explorer into scenes.
• refactor the actor creation to the MAbstractActorFactory by creating static methods and calling these instead of creating actors in various UI classes

Functionality provided and to test:

• Drag/Drop actors from the toolbar into any scene
• Drag/Drop the load button from the toolbar to any scene
• Drag/Drop actor files from file explorer to any scene
• Click actors or the load actor button to create an actor in the last scene interacted with
• All of the above with "Alt"-Modifier opens a window to rename the actor and select a set of scenes to add the actor to.

Also since some of the actor creation functionality had some actor specific checks (e.g. the trajectory actor asks for a sync control), this should also be tested with these actors. This merge request already uses the file utility classes ( !272 (merged)).

Resolves #321 (closed)