Finish preliminary offline support

* ProgressProvider now has an abort() call
* Abort button support added to the progress dialog
* YggdrasilTask and MojangAccount adapted to support abort

YggdrasilTask will time out after 10 seconds of no network activity, or when the user pushes the Play Offline button.
In offline mode, all instance update tasks are skipped! This will need further work.

logic/auth/flows/RefreshTask.cpp

@@ -143,9 +143,9 @@ QString RefreshTask::getStateMessage(const YggdrasilTask::State state) const
 	switch (state)
 	{
 	case STATE_SENDING_REQUEST:
-		return tr("Refreshing: Sending request.");
+		return tr("Refreshing login token.");
 	case STATE_PROCESSING_RESPONSE:
-		return tr("Refreshing: Processing response.");
+		return tr("Refreshing login token: Processing response.");
 	default:
 		return YggdrasilTask::getStateMessage(state);
 	}

logic/auth/flows/ValidateTask.h

@@ -13,6 +13,10 @@
  * limitations under the License.
  */
 
+/*
+ * :FIXME: DEAD CODE, DEAD CODE, DEAD CODE! :FIXME:
+ */
+
 #pragma once
 
 #include <logic/auth/YggdrasilTask.h>