Flatten GUI tree somewhat

The very deep tree structure didn't really aggree with me, sorry. This
makes the core module rather large, but on the other hand that just
highlights that it is rather large.

gui/syncthing/core/upgradingDialogView.html

@@ -0,0 +1,3 @@
+<modal id="upgrading" icon="refresh" title="{{'Upgrading' | translate}}" status="info">
+	<p><span translate>Syncthing is upgrading.</span> <span translate>Please wait</span>...</p>
+</modal>