fix(windows): make keymanx64 responsible for its own lifecycle

Fixes #4976.
Fixes KEYMAN-WINDOWS-2K.
Fixes KEYMAN-WINDOWS-3F.
Fixes KEYMAN-WINDOWS-5A.
Fixes KEYMAN-WINDOWS-3E.

This fix reworks keymanx64's lifecycle, moving responsibility for
process shutdown from keyman.exe to keymanx64.exe.

This eliminates the need for interprocess communication, and simplifies
the startup and shutdown of keymanx64.exe. Removing this means that we
can more safely handle situations where two instances of keymanx64 may
be started, as one of them will rapidly terminate when it discovers that
its parent process has disappeared.