Streamline creating tile atlas sources

[KoBeWi]

Part of godotengine/godot-proposals#7177

• open file dialog when creating atlas source
• automatically change to Setup tab when new source is created

godot.windows.editor.dev.x86_64_x30OWjZuRI.mp4

As a side-effect, creating empty (no texture) atlas sources is no longer possible.

[groud]

As a side-effect, creating empty (no texture) atlas sources is no longer possible.

I am a bit worried about that, but I don't think there's a real situation where that would matter much. I guess it's acceptable then.

Worst case we could add another popup asking something like "You did not select a texture, create an atlas source without texture anyway ?". But that's if anyone complains.

Looks good anyway, thanks!

[YuriSizov]

Thanks!