NodeManager rework and many fixes

A lot of fine grinding and small fixes, and I did a big rework of the NodeManager.

NodeManager changes

• I reorganized some internal processing and made it much cleaner
• it also has source code dialogs now to directly edit the metacodes
• it detects changes in the original source file while editing inside, warns the user, and lets him choose whether to override those or not
• I updated the stylesheets, now using the same as Ryven
• a lot of small adjustments, enhancements, and improvements to significantly support the workflow

Ryven changes

• added a function for using method references in metacode files to enable successful method editing in Ryven
• changed the source code editing info dialog
• fixed a package bug in the val node
• added an API method for NodeInstance to provide direct access to the window's stylesheet
• changed the structure of saved points in DrawingObjects (Ryven can still load projects using the old signature though)