Split the User Actions Menu out of Workspace

All methods and variables related to the User Actions Menu
(rmb window deco, Alt+F3) is moved out of the Workspace class
into an own UserActionsMenu class.

The class needs only a very small public interface containing
methods to show the menu for a Client, closing the menu and
discarding the menu. Everything else is actually private to the
implementation which is one of the reasons why it makes sense
to split the functionality out of the Workspace class.

As a result the methods and variables have more sane names and
the variable names are standardized.