Store database icons as QImage instead of QIcon.

This has the advantage that they can be used without a running X server.
Add methods to retrieve QPixmaps that are converted from the stored QImages
and cached by QPixmapCache.

src/format/KeePass2XmlWriter.h

@@ -21,7 +21,7 @@
 #include <QtCore/QDateTime>
 #include <QtCore/QXmlStreamWriter>
 #include <QtGui/QColor>
-#include <QtGui/QIcon>
+#include <QtGui/QImage>
 
 #include "core/Database.h"
 #include "core/Entry.h"
@@ -46,7 +46,7 @@ private:
     void writeMetadata();
     void writeMemoryProtection();
     void writeCustomIcons();
-    void writeIcon(const Uuid& uuid, const QIcon& icon);
+    void writeIcon(const Uuid& uuid, const QImage& icon);
     void writeCustomData();
     void writeCustomDataItem(const QString& key, const QString& value);
     void writeRoot();