Fixes #670 Theme Upgrading issues

#670
Also fixes #215 Stylesheets may not have a disporder

The upgrader now
- adds all new stylesheets to the master theme and sets the disporder according to `mybb_theme.xml`
- goes through every theme and assigns every stylesheet a disporder
- adds the new color stylesheets to the default theme

The stylesheets in the default theme may not have the same disporder as the ones in the master theme. But I think that's an issue which doesn't need to be fixed.
Should be `stylesheet(s)`?