fix(windows): Change TLangSwitchRefreshWatcher ownership

Fixes #4751.
Fixes KEYMAN-WINDOWS-76.
Fixes KEYMAN-WINDOWS-79.
Fixes KEYMAN-WINDOWS-7B.
Fixes KEYMAN-WINDOWS-7E.
Fixes KEYMAN-WINDOWS-7F.
Fixes KEYMAN-WINDOWS-7K.
Fixes KEYMAN-WINDOWS-7N.
Fixes KEYMAN-WINDOWS-80.

This resolves an issue where the TLangSwitchRefreshWatcher thread would
terminate early on Win7 (as it had no work to do), and free itself, but
then the main form would reference it post-free. Responsibility for free
has been moved to the main form.

This would result in various exceptions as state is corrupted at this
point.