QmlProjectManager: Disable by default

It's not the preferred way of operation nowadays and harms startup
times. People who find the plugin useful can still enable it.

Change-Id: I8328f8e8085103d4469bbd7016b86218b4b238b7
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/qmlprojectmanager/QmlProjectManager.json.in

@@ -2,6 +2,7 @@
     \"Name\" : \"QmlProjectManager\",
     \"Version\" : \"$$QTCREATOR_VERSION\",
     \"CompatVersion\" : \"$$QTCREATOR_COMPAT_VERSION\",
+    \"DisabledByDefault\" : true,
     \"Vendor\" : \"Digia Plc\",
     \"Copyright\" : \"(C) 2014 Digia Plc\",
     \"License\" : [ \"Commercial Usage\",