Move heavy SystemProperties loads to an opt-in model

This avoids heavyweight operations when we're not actually running config checks.

app/gui/StreamSegue.qml

@@ -4,6 +4,7 @@ import QtQuick.Window 2.2
 
 import SdlGamepadKeyNavigation 1.0
 import Session 1.0
+import SystemProperties 1.0
 
 Item {
     property Session session
@@ -120,6 +121,10 @@ Item {
         session.sessionFinished.connect(sessionFinished)
         session.readyForDeletion.connect(sessionReadyForDeletion)
 
+        // Ensure the SystemProperties async thread is finished,
+        // since it may currently be using the SDL video subsystem
+        SystemProperties.waitForAsyncLoad()
+
         // Kick off the stream
         spinnerTimer.start()
         streamLoader.active = true