Utils: Join SynchronousProcess::run and runBlocking implementations

Make functionality dependent on an (intentionally ugly)
setProcessUserEventWhileRunning call.

Also, back-paddle a bit on API combination of QtcProcess and
SynchronousPrceoss for now and prevent the QtcProcess-and-
runBlocking and SynchronousProcess-and-start combinations.

Goal is still to have all in QtcProcess in the end, but this
may take a while.

Change-Id: Ic146ec5db0ab8dc9613e5b2af5f4dc90bc7465ca
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/perforce/perforceplugin.cpp

@@ -1273,7 +1273,8 @@ PerforceResponse PerforcePluginPrivate::synchronousProcess(const QString &workin
     }
     process.setTimeOutMessageBoxEnabled(true);
     process.setCommand({m_settings.p4BinaryPath.value(), args});
-    process.run();
+    process.setProcessUserEventWhileRunning();
+    process.runBlocking();
 
     PerforceResponse response;
     response.error = true;