Refactored rendering of textures for save and exporting material now saves all textures.
Added an option to start a clear the project, and proper "save" and "save as..." options.
Added support for disconnects.
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)
