Blender design, fixes, code cleanups, and new input widget signature

• Ryven has now a Blender design!
• I updated a few internal signatures like the remove_event() method in NodeInstances and widgets
• I changed the way custom input widgets are specified (no widget_type argument anymore)
• I fixed some nodes and widgets and added standard line edits including self resizing ones
• I changed the widgets of a few nodes now using resizing and or borderless line edits which looks way nicer
• I moved the package_path for widgets outside of their class (useful for imports of custom classes etc.)
• there is a new Extract Property node which is really useful, an example is included in the examples project
• and I changed the file extensions to .rpo for projects and .rpc for packages