{Fancy,Debugger}MainWindow: Consolidate dock actions menu population

There are several places where the list of dock actions are shown,
and at least two where it was created.

Change-Id: Ib2c18e602b5d6f57c1b7471bd75f3b989d536600
Reviewed-by: Eike Ziller <eike.ziller@digia.com>