Update UI libraries and lots of small stuff

Ok, this is going it be another fun one. Please test thoroughly as changing so much is going to break stuff, thanks.

What has been done:
  - #199: updated jQuery, jQuery UI, Bootstrap and used plugins. Replaced combobox plugin.
  - #109: as discussed only partly related, the actual changes are removing the manual language switches we simplifies I18N a lot. Works for me now
  - #206: fixed
  - #208: did not test beforehand, but works now for me
  - #207: fixed
  - #186: fixed
  - #143: fixed
  - #110: had already a confirmation, now probably more prominent
  - #67: please re-test, works for me, have no idea about before

Hope not too much is still broken :)