Breaking changes include:
* Rename of GEditorModeTools -> GLevelEditorModeTools to signify that it applies only to the level editor modes
* Addition of FEditorModeRegistry, responsible for managing and creating new editor modes. Modes are no longer registered with an instance of the mode, instead with a mode factory that is able to create a new mode of that type.
* Editor modes now operate on FEditorViewportClients rather than FLevelEditorViewportClients
* Added ability to specify an FEditorModeTools when creating an FEditorViewport
Moved component vizualiser manager handling outside of individual editor modes, and into FLevelEditorViewportClient. This should make it easier to transplant in future.
This work addresses TTP#334640 - EDITOR: Investigate making editor modes a per-'editor' concept
Reviewed by Michael Noland, Matt Kuhlenschmidt
[CL 2109245 by Andrew Rodham in Main branch]
TTP# 279062 - ROCKET: TASK: PUNTABLE: Viewports: "Drag to Attach" feature
Moved FEdModePicker out of the Private section of the PropertyEditor module, and into its own module so that it can be shared between the PropertyEditor and LevelEditor modules. Also renamed it to FEdModeActorPicker.
Added a way to filter FEdModeActorPicker by checking the actor directly (this complements the class type check it already had).
Added an actor picker button next to the scene outliner in the "Attach To" menu on actors. This is also mapped to the Alt+A command.
Built and tested in unity and non-unity.
ReviewedBy Thomas.Sarkanen, Max.Preussner
[CL 2060928 by Jamie Dale in Main branch]
