More generic debug/test option handling

  KRB5_AUTH_DIALOG_DEBUG_NO_APP_MENU

is now

  KRB5_AUTH_DIALOG_DEBUG=no-app-menu

This will allow to extend this to more options
Add no-header-bar test option

This allows us to test the dialogs without header bars
Add no-persistence test option

This allows us to test notification systems without persistence.
We can then e.g. embed it into an Xnest sessions's xfce4-panel.
Build context menu from GActions too

to get rid of the open coded menu. This allows us to use the same
functions for the app menu and the context menu.