Change `Window` from a borrowed handle to a weakly owned handle

[prokopyl]

This PR implements the idea discussed with @glowcoil on Discord a few days ago, which is to make the Window handle 'static and clone-able, so that it can be easily stored in the WindowHandler.

Note this PR is based on #172 for now, which is why the new example shows up in the diff. This can be changed depending on #172's merge status. This has been properly rebased now.

This makes the following breaking API changes, for all platforms and backends:

• Removes the lifetime of the Window type.
• The Window type is now an owned, weak handle to the actual underlying window.
• Makes the Window type clone-able.
• The Window argument passed to the WindowHandler builder is now a fully owned value instead of a temporary borrow, which enables the handler to store it for its whole lifetime.
• Removes the Window argument on WindowHandler::on_frame and WindowHandler::on_event methods.
• All Window methods now only take &self instead of &mut self.
• All Window methods are now internally fallible: they will panic if the actual window the handle points to has been destroyed. This can only happen if the WindowHandler misbehaves and clones the Window outside of its own lifetime.
• The GlContext has received the same treatment, for the same reasons: it's now a weak, owned clone-able handle to the GL context of a window, and its methods will also panic if the underlying context is destroyed.

As of now, this PR is still a WIP:

• [x] The X11 backend is complete and functional, but it will probably need to be rebased on top of #173 before this PR can be merged. It's done!
• [x] I started working on the Win32 backend, but it's not done yet. Done!
• [ ] I haven't touched the Cocoa backend, and it will not build at all at the moment. Done, but still needs testing.

As of now, this PR will not build on macOS or Windows until I've completed the work mentioned above.

[prokopyl]

Win32 backend update done, tested and functional! :slightly_smiling_face: