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

app/gui/main.qml

@@ -56,13 +56,10 @@ ApplicationWindow {
                 wow64Dialog.open()
             }
 
-            if (SystemProperties.unmappedGamepads) {
-                unmappedGamepadDialog.unmappedGamepads = SystemProperties.unmappedGamepads
-                unmappedGamepadDialog.open()
-            }
-
-            // Hardware acceleration is checked asynchronously
+            // Hardware acceleration and unmapped gamepads are checked asynchronously
             SystemProperties.hasHardwareAccelerationChanged.connect(hasHardwareAccelerationChanged)
+            SystemProperties.unmappedGamepadsChanged.connect(hasUnmappedGamepadsChanged)
+            SystemProperties.startAsyncLoad()
         }
     }
 
@@ -77,6 +74,13 @@ ApplicationWindow {
         }
     }
 
+    function hasUnmappedGamepadsChanged() {
+        if (SystemProperties.unmappedGamepads) {
+            unmappedGamepadDialog.unmappedGamepads = SystemProperties.unmappedGamepads
+            unmappedGamepadDialog.open()
+        }
+    }
+
     // It would be better to use TextMetrics here, but it always lays out
     // the text slightly more compactly than real Text does in ToolTip,
     // causing unexpected line breaks to be inserted