Move it to the mozilla::widget namespace.

Use enum classes for transparency, popup type, popup level, etc.

Mostly automated with sed, but there were a few manual changes required
as well in windows code because they relied on Atomic<TransparencyMode>
working (which now doesn't because TransparencyMode is 1 byte instead of
4 bytes).

Differential Revision: https://phabricator.services.mozilla.com/D167537