Improved MAVLink dialect selection.

MAVLink dialects can now be specified as a list, so multiple dialects can be compiled in simultaneously (may result in conflicts between dialects).
MAVLink dialects are checked for existance before support is added.
Users are notified of which dialects are used and which cannot be added and why.
The README now has info about specifying MAVLink dialects.

QGCExternalLibs.pri

@@ -119,6 +119,7 @@ else {
 	    MAVLINK_DIALECTS += $$dialect
 	    message($$sprintf("Using default MAVLink dialect '%1'.", $$dialect))
 	} else {
+
 	    warning($$sprintf("Default MAVLink dialect '%1' does not exist at '%2'!", $$dialect, $$MAVLINKPATH_REL))
 	}
     }