KolabMailSynchronizer: rework of folder walking on info_sync()
KolabMailSynchronizer: rework of folder walking on info_sync()

* new static function for walking folders and updating
  InfoDb metadata
  (kolab_mail_synchronizer_folders_update_walk())
* called in kolab_mail_synchronizer_folders_update_infodb()
  (replaces three similar code segments)
* set folder type and context on mail summaries in
  kolab_mail_synchronizer_uids_infosync()

GNOME/evolution-kolab,GNOME/evolution-kolab