New GIF export process

[novhack]

• Remove godot-gifexporter submodule/plugin.
• Add jegor377/godot-gdgifexporter entirely written in GDScript instead.
• Add exporting of animation frames in a separate thread and an export progress bar (so Pixelorama doesn't freeze during export).
• Remove background color option from gif export since we now support transparency - color (0,0,0,0) is used as transparent.

Massive shout out to jegor377 who wrote the exporter and Kinwailo who created median-cut quantization method and optimized most of the code.

Original PNG:

Exported GIF with 256 colors on Ryzen 2600 in 8 seconds (most likely one of the worse cases):

New export progress bar:

[OverloadedOrama]

Looks really good! Amazing work guys, thank you!