Move main window management to new class

[michael2402]

Create a new Class MainFrame for the main window, move code from Main and MapView there.

This class should do:

• The title management
• Basic geometry setup
• Automatically display the start screen as long as no layers are present.
• Do all MapFrame handling

Long term goals:

• Remove Main.gettingStarted
• Do not destroy the MapFrame if all layers are removed - keep an empty layer list. This would remove many Main.map == null checks and prevent problems for missing checks.

We should get away from the concept that the UI will always be structured the way it is now.

Patches: #12927