+ sfx2::sidebar::EnumContext moved to the vcl module
+ TabPage contains vector with context values
+ vcl builder reads control's contexts from the "class" mark
+ ContextTabControl shows tabs depending on context

Change-Id: I661b0d3f35d46ace2a2e8eb1d374148f0c60017d
Reviewed-on: https://gerrit.libreoffice.org/26447Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>