+ added sfxlo-ContextVBox
+ notebookbar's .ui file must contain control
  implementing NotebookbarContextControl interface
  with id "ContextContainer"

Change-Id: Ice81e23c4ba742564ebceeda95be120ea3f58c99
Reviewed-on: https://gerrit.libreoffice.org/28247Tested-by: Jenkins <ci@libreoffice.org>
Tested-by: Yousuf Philips <philipz85@hotmail.com>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>