Add ability to show in File System dock non-supported file type

[qarmin]

Godot version:
3.1 master

OS/device including version:
Windows 10

Issue description:
Godot should have an option to show files with non-supported file type, of course not by default, for example .kra, .odt etc.
Should be prepared special icon which will easily show to user which file type are not supported by engine.

[Calinou]

Closing in favor of godotengine/godot-proposals#677, as feature proposals are now tracked on the Godot proposals repository.