Themes 3: Intermission: Appearance Tab and fixes

Done:

• reorganized settings a bit, created "appearance" tab to put all the visual (and not really functional) stuff in there.
• fixed firefox+redmond theme bug
• added UI scaling options
• added UI roundness options
• added emoji scale options
• fix existing theme editor's preview pane
• many fixes to emoji picker, now scalable with rest of emoji, further customizations of it - later
• themes selector with preview
• add option to make underlay solid or transparent
• add option to override font