Rewrite FileDialog in C++ && add setting to force async load timelime.

Also fixes repeat section label and empty space when message is redacted.

Fixes #27

qml/form/SettingGeneralForm.qml

@@ -3,12 +3,18 @@ import QtQuick.Controls 2.2
 
 Page {
     property alias lazyLoad: lazyLoadSwitch.checked
+    property alias asyncMessageDelegate: asyncMessageDelegateSwitch.checked
 
     Column {
         Switch {
             id: lazyLoadSwitch
-            text: "Lazy Load at Initial Sync"
+            text: "Lazy load at initial sync"
         }
+        Switch {
+            id: asyncMessageDelegateSwitch
+            text: "Force loading message delegates asynchronously"
+        }
+
         Button {
             text: "Invoke GC"
             highlighted: true