Draft: spinner: add style class when animations are disabled

CSS animations are stopped when the gtk-enable-animations setting is disabled, which makes the spinners look stuck, as if the whole app UI was frozen and unresponsive. This gives an wrong visual feedback to the user.

This MR automatically adds a .frozen style class to spinners when animations are disabled. This allows themes to use a fallback static icon, in place of the typical animated one.

For reference, Firefox replaces the loading-tab bouncing indicator by a static hourglass icon when animations are disabled.

Remaining points:

• add a proper fallback icon (hourglass?) instead of document-open-recent-symbolic