Bug 1906037 - Restore macOS desktop tinting on dialog windows. a=RyanVM

After bug 1867854, dialog windows on macOS no longer show desktop tinting on the window background. Since this requires a transparent Gecko background, the `-moz-mac-unified-toolbar-window` appearance added in bug 1870481 should be applied to every window. It has been renamed to reflect this.

Original Revision: https://phabricator.services.mozilla.com/D215596

Differential Revision: https://phabricator.services.mozilla.com/D219588