tools: deprecate gst-xmllaunch and print fat warning if someone tries to use it

Pipeline serialisation to and from XML is horribly broken for all
but the most simple use cases, and will likely never be fixed.
Make sure everyone playing around with these tools is aware of
this, to avoid frustration. See countless bug reports in bugzilla.

tools/gst-launch.c

@@ -779,6 +779,10 @@ main (int argc, char *argv[])
   memcpy (argvn, argv + 1, sizeof (char *) * (argc - 1));
 #ifndef GST_DISABLE_LOADSAVE
   if (strstr (argv[0], "gst-xmllaunch")) {
+    /* FIXME 0.11: remove xmllaunch entirely */
+    g_warning ("gst-xmllaunch is deprecated and broken for all but the most "
+        "simple pipelines. It will most likely be removed in future. Don't "
+        "use it.\n");
     pipeline = xmllaunch_parse_cmdline ((const gchar **) argvn);
   } else
 #endif
@@ -812,6 +816,10 @@ main (int argc, char *argv[])
   }
 #ifndef GST_DISABLE_LOADSAVE
   if (savefile) {
+    g_warning ("Pipeline serialization to XML is deprecated and broken for "
+        "all but the most simple pipelines. It will most likely be removed "
+        "in future. Don't use it.\n");
+
     gst_xml_write_file (GST_ELEMENT (pipeline), fopen (savefile, "w"));
   }
 #endif

tools/gst-xmllaunch.1.in

@@ -23,6 +23,10 @@ You can pass "-" as the XML\-FILE to read from stdin.
 XML\-FILE can be a URI as well, thanks to the wizardry of libxml. I'm not really
 sure what all is supported, it seems http works just fine though.
 
+.
+.SH "WARNING"
+\fIgst\-xmllaunch\fP is deprecated and broken for all but the most simple
+pipelines. It will most likely be removed in future. Don't use it.
 .
 .SH "OPTIONS"
 .l