mainwindow: prepared for multiple edit boxes

the edit box context menu actions are now saved in a member variable so
that the action list can be reused in another edit box instance and the
separators were given the main window instance as parent instead of the
edit box instance

the connection of the signals from the edit box instance and the setting
of edit box context menu were moved to a new set active function, which
can be called for any edit box instance, with preliminary code to
disconnect the signals and remove the context menu actions of a previous
edit box (commented since only one edit box is currently supported)