Preferences UX: use traditional OK, Cancel, Apply (and Reset to Defaults) buttons

[skyjake]

Changes in Preferences get applied quite inconsistently at the moment: some settings are applied immediately when toggled, and some (like the UI scale) are applied when the dialog is closed, and others require reloading the page.

On desktops, it would be better to have traditional OK, Cancel, and Apply buttons. Also a Reset to Defaults would be useful.

With #191, Preferences should be a detached top-level window of its own.

[skyjake]

v1.15 makes Preferences an independent top-level window.