QmlDesigner: Refresh preview image every hour

It is to assumed that the preview image will be changed and we don't need
to collect the time stamps of the files.

Task-number: QDS-5924
Change-Id: Icf5540d7bcc9da17a1497641f6189f35eb47f5d2
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/libs/sqlite/sqlitetimestamp.h

@@ -41,6 +41,17 @@ public:
     }
 
     friend bool operator!=(TimeStamp first, TimeStamp second) { return !(first == second); }
+    friend bool operator<(TimeStamp first, TimeStamp second) { return first.value < second.value; }
+
+    friend TimeStamp operator+(TimeStamp first, TimeStamp second)
+    {
+        return first.value + second.value;
+    }
+
+    friend TimeStamp operator-(TimeStamp first, TimeStamp second)
+    {
+        return first.value - second.value;
+    }
 
     bool isValid() const { return value >= 0; }