GSoC Phase 2: Minimal K12 mode

What this merge request does

• Provides minimal, working K12 Mode when Purr Data is started with the -k12 flag.

Known bugs

• The z-order problem is not yet solved.
• User now needs to manually copy the K12 folder (located at l2ork_addons/K12 in the repo into the @extra folder, because no changes were made to the building process.
• (On Mac) User needs to cd Pd-l2ork.app/Contents/Resources/app.nw && ./pd-l2ork -k12 (i.e. the pwd needs to be in app.nw, otherwise all external libraries won't get loaded.