This PR overhauls the GUI backend selection, making it clearer (I hope), improving support for various use-cases, and making it easier to add support for specific use-cases.

• When on IPython, honor the selected %gui.
• Prevent Qt backend from running the event loop if it's already made active by the env.
• If multiple Qt libs are imported, check if any of them has an app object.
• The above also makes that Qt is selected in $ ipython --gui qt.
• Prefer glfw when we detect a running asyncio loop.
• The above makes wgpu interactive in e.g. $ ptpython --asyncio and $ python -m asyncio.
• Users can set WGPU_GUI_BACKEND to force a backend. Though WGPU_FORCE_OFFSCREEN still exists to overload during tests.
• Docs on interactive use.
• Can call run() multiple times (for both qt and glfw).
• Fix that for the glfw backend, obtaining an asyncio event loop fails on e.g. ipython.