Add support for persistent variables in theme's <variables> section

* Add support for variables with persist="1", which will cause
  them saved into settings file and reloaded on next boot.
* This is handy for theme makers, who can use this to
  implement "theme settings".

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>

Change-Id: I6415979011b38cbb15dec9909c445b02da6eadc0