Icon cleanup

Finally some icon cleanup:
* differentiate delete (destructive red) from close icon, much needed! And no extra hover state anymore.
* use consistent icon for restore/versions/history cc @schiesbn 
* use proper style for add icon cc @tanghus 
* use only one loading spinner! 32px, scale in CSS where needed cc @zimba12
* remove unused images, and some smaller fixes

Please review @owncloud/designers! Especially the loading spinner size change might cause problems here and there which I didn’t anticipate yet, but will be easily fixed.