sessionMode: Add hasRunDialog property

Add a sessionMode.hasRunDialog property, which determines whether
the keyboard shortcut to access the run dialog should be active
or not.

https://bugzilla.gnome.org/show_bug.cgi?id=676156

js/ui/sessionMode.js

@@ -14,6 +14,7 @@ const _modes = {
              allowSettings: false,
              allowExtensions: false,
              allowKeybindingsWhenModal: true,
+             hasRunDialog: false,
              sessionType: Shell.SessionType.GDM },
 
     'user': { hasOverview: true,
@@ -22,6 +23,7 @@ const _modes = {
               allowSettings: true,
               allowExtensions: true,
               allowKeybindingsWhenModal: false,
+              hasRunDialog: true,
               sessionType: Shell.SessionType.USER }
 };