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>