Eliminate C++ libp3showbase library

[rdb]

The C++ libp3showbase library (part of panda3d.direct) contains only a small handful of functions to augment the Python ShowBase code. However, many of these functions are either unused or unnecessary, or are better placed at a more appropriate location in the source tree. It would be a fairly minimal amount of effort to move these functions and eliminate this module entirely.

• [ ] init_app_for_gui() — only calls activate_osx_application(); test if still necessary and if so, move to cocoadisplay
• [ ] activate_osx_application() — never called directly, see above
• [x] get_particle_path() — can be moved to C++ particle module, defined in Python, or we can decide to use getModelPath()
• [ ] store/allow_accessibility_shortcut_keys() — can be automatically handled by windisplay instead
• [x] add_fullscreen_testsize() and friends — apparently unused entirely
• [x] throw_new_frame() — can be replaced with the following Python code:

EventQueue.getGlobalEventQueue().queueEvent("NewFrame")