gdm: Move logo into the panel

GDM has a 'logo' key in its schema to allow distributors to add
some branding. It is currently placed above the user list, which
no longer works too well since the login screen lost its dialog
window. Display the logo in the top-left corner instead of the
Activities button instead.

https://bugzilla.gnome.org/show_bug.cgi?id=685852

js/ui/sessionMode.js

@@ -39,7 +39,7 @@ const _modes = {
         unlockDialog: imports.gdm.loginDialog.LoginDialog,
         components: ['polkitAgent'],
         panel: {
-            left: [],
+            left: ['logo'],
             center: ['dateMenu'],
             right: ['a11y', 'display', 'keyboard',
                     'volume', 'battery', 'powerMenu']