[WIP] Adds texture2d stub property

[imothee]

Description

Adds a texture2d property to Pandora that properly handles serializing AtlasTextures (since the default resource type requires a resource to exist on disk whereas this can re-use a spritesheet path and a region).

Addressed issues

Could not save AtlasTextures accurately.

Screenshots

Shows the added icon to the property bar

Shows the default dropdown resource picker

Shows dragging an existing Texture2D in from another template resource

[bitbrain]

@imothee any progress on this? The tests are failing. Btw, you only gotta support Godot 4.2 onwards, no need any longer to remain backwards compatible with Godot 4.0/4.1

[imothee]

I hadn't spent any time on this since I was waiting/chatting in the Godot discord about if/when they'd expose the texture selector to general scripting. I'm in Australia travelling so I'm a bit behind but I can take a look at this when I'm back home and see where things are