+ added menu controller for .uno:Notebookbar showing
  all implementations
+ switching between .ui files
+ officecfg stores implementation description
+ added second implementation file notebookbar_groups.ui

Change-Id: I9f406daced7a111793f61a2e9f939cba27d92184
Reviewed-on: https://gerrit.libreoffice.org/28067Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>