[windows] Nop WM_ERASEBKGND to avoid potential flickering.
[windows] Shorten `Message` to `Msg` in butane_window_update.
[windows] Remove notes about refactoring _WindowProcW.

It's better to keep all the message handling code in one place, rather than moving most of it into butane_window_update. Additionally, events will be queued through the the butane_window_t instance anyways--removing the need for functionalizing the code in _WindowProcW.
[windows] Use `hWnd` rather than `window->hndl` in WM_CLOSE.
[windows] Return a non-zero value (TRUE) after handling a message.
[windows] Stub WM_NCCREATE, WM_CREATE, and WM_DESTROY handlers.
[windows] Note that WM_GETICON should be implemented.
[windows] Note various messages that should be handled.