Fix a crash that occurs when library "KF5WidgetsAddons" cannot be loaded

Add a check to see if the library "KF5WidgetsAddons" could be loaded and
return if that's not the case.

Also move a using declaration near the place where it is used first.

src/gui/MainWindow.cpp

@@ -69,8 +69,13 @@
 //Work around an issue on KDE5 as per https://bugs.kde.org/show_bug.cgi?id=337491#c21
 void disableAutoKeyAccelerators(QWidget* mainWindow)
 {
-	using DisablerFunc = void(*)(QWidget*);
 	QLibrary kf5WidgetsAddon("KF5WidgetsAddons", 5);
+    if (!kf5WidgetsAddon.isLoaded())
+    {
+        return;
+    }
+
+    using DisablerFunc = void(*)(QWidget*);
 	DisablerFunc setNoAccelerators = 
 			reinterpret_cast<DisablerFunc>(kf5WidgetsAddon.resolve("_ZN19KAcceleratorManager10setNoAccelEP7QWidget"));
 	if(setNoAccelerators)