win32: support key modifiers (shift, ctrl, alt)

Support for this is rather simple, and some combinations of modifiers
and keys don't work. For example, Ctrl+Alt+character is not supported,
because Windows doesn't emit a WM_CHAR in this case.

Also add support for the pause and print screen keys. Remove the
pointless KEY_CTRL translation. Remove KEY_CTRL altogether, because it
was not clear what it was actually supposed to mean.