Added support for opening images in an already-running RT instance

This is enabled by the new -R switch of rawtherapee.
The idea is that you first start the "server" with

  `$ rawtherapee -R`

and then open images on this server by using

  `$ rawtherapee -R /path/to/raw.file`

Currently disables on OSX (there seems to be some "legacy" support for this in
rtwindow, but I have no way of testing it and/or integrating it with this at
the moment)

rtgui/main-cli.cc

@@ -59,7 +59,7 @@ Glib::ustring argv0;
 Glib::ustring creditsPath;
 Glib::ustring licensePath;
 Glib::ustring argv1;
-bool simpleEditor;
+//bool simpleEditor;
 //Glib::Threads::Thread* mainThread;
 
 namespace