Enable hidpi rendering of titlebar buttons when the theme file wants it.

- Incremented major version to 4 (mainly so we don't break our mint theme
  in Mate) - themes are treated identically whether they're -3 or -4.
- image files in the theme folder are prepended to the icon theme,
  and if they are svgs, they will be scaled for hidpi if necessary, which
  will prevent pixelation.
- all other themes should work un-modified, and scale correctly, but there could
  be pixelation.