Updates and fixes for GFLT Import

[mr-phazer]

Update and fix to GLTF import

Importing of GLTF files was not functioal and/or it lacked maany features

I have essentially copied the system Ole made for GLTF EXPORT, and used it for IMPORT, incluidig UI = an import dialog box.
The import dialog is a different style, and I intend to change the export dialog to follow this style, too

This is the functionality that is added/fixed:

Images and description of addistions

User selects file and picks menu item from context menu:

"Advanced Import"
Fixed: Issue where you could use "avance import" on the pack root node

AE Shows:

User is presented with "Open File", user select a file to import

AE Shows newly designed import dialog

AE imports, converts to RMV2, user opens result in viewer.

Notes

• If one export/imports witthout changing settings it should work flawlessly. But there might be compinations of settings that
  does not work properly
• are logically non-sensiccal / illegal, but the user is not prevent from clicking "import"
• almost all changes should be inside the ImportExport branch, aside from the bug-fix for the root note node import menu fix
• More Unit Tests should be added
• Some real-world testing would be good