x11: use different strings for WM_CLASS depending on context
Created by: Hinsbart
Fixes #766 (closed), enables the use of custom window rules for tiling WMs. (eg. run editor maximized, and games floating/centered)
Sets the resource name in the WM_CLASS property to:
- "Godot" for game windows (same as before)
- "Godot_Editor" for the editor
- "Godot_ProjectList" for the project manager