API revamp

[brentyi]

Before:

server.add_gui_button()
server.add_gui_folder()
server.on_scene_click()
server.add_mesh()
server.reset_scene()

After:

server.gui.add_button()
server.gui.add_folder()
server.scene.on_click()
server.scene.add_mesh()
server.scene.reset()

Wrappers + deprecation warnings should be added if we decide to go with this change.