+ extended vcl builder to parse priority
+ IPrioritable interface for controls with priorities
+ added IPrioritable as a base for VclContainer
+ Added PriorityHBox - box which shows controls if we have enough space
  PriorityHBox listen vcl events from SystemWindow to detect Resize

Change-Id: I74ac1a80e7d0a061f5e7a8584dbb2abf956053c7
Reviewed-on: https://gerrit.libreoffice.org/26983Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>