Let themes predefine slidebars

[JoryHogeveen]

Currently there is no (API) way for themes to add slidebars on load automatically so users won't need to add them manually (especially when using the same theme more often this can be a drag).
A simple API function to add them auotmatically would be very handy.

Things to consider:

• Should it be editable or are the theme parameters fixed?
• If editable the slidebar can only be loaded once, after that it's part of OCS and the user can modify it.
  • This also disables any automatic updates that could be done with the theme...
• Implement an "editable" parameter for slidebars. No need to add a checkbox in the UI but it could be handy to add it to the list of slidebars at half (or so) opacity.