now load/save dialog use the non-editor classes (thus usable when running the project).
Refactored graphedit and node code (mainly load/save and adding and removing nodes).
Added blend modes (Dissolve, Multiply, Screen, Overlay, Hard Light, Soft Light).
Fixed color popup position in the GradientEditor.
Fixed initial panoram shown in preview.
Now gradient editor can remove points.
Improved preview:
- added panorama to 3d preview (with panoramas from the godot material demo)
- texture preview can be maximized
Added Image node that can be used to import textures
Renamed all get_shader_code methods to _get_shader code so a caching system can be integrated in node_base (and we can get rid of get_source_rgb and get_source_f functions)
