New settings: preferred fonts and top bar margin, improvements

Complete changelog:
* New preference: preferred fonts, which allows to specify custom fonts and webfonts.
  It is a comma-separated list, with quotes around each font name. Webfonts are specified using the URL|Font name syntax.
* New preference: top bar margin, which determines the margin of the top bar (obviously). It is only used when the 'fixed top bar' option is enabled, and the default value is 55px.
* The CustomLangsAPI cache is now cleared automatically if the revision of the ProTheme in the local storage does not match the one in the code. This is a temporary workaround and it will be implemented directly in the CustomLangsAPI.
* Various styling fixes, including the fix to the issue reported by Marval13 about bigger font sizes.
* The thumbs are no longer in italic (I still don't know why they were in italic tags).