Allow gramplets to be displayed in the dashboard only

Add a new navigation type of 'Dashboard' in gramplet definitions.
Define the dashboard To Do gramplet to be dashboard-only.

gramps/gui/widgets/grampletpane.py

@@ -1429,7 +1429,8 @@ class GrampletPane(Gtk.ScrolledWindow):
                 qr_menu = ag_menu.get_submenu()
                 qr_menu = Gtk.Menu()
                 names = [gplug.name for gplug in PLUGMAN.get_reg_gramplets()
-                         if gplug.navtypes == []]
+                         if gplug.navtypes == []
+                            or 'Dashboard' in gplug.navtypes]
                 names.sort()
                 for name in names:
                     add_menuitem(qr_menu, name, None,

gramps/plugins/gramplet/gramplet.gpr.py

@@ -277,6 +277,7 @@ register(GRAMPLET,
          gramplet_title=_("gramplet|To Do"),
          version="1.0.0",
          gramps_target_version=MODULE_VERSION,
+         navtypes=["Dashboard"],
          )
 
 register(GRAMPLET,