Rename headers to end with _p.h

Frameworks have a convention of naming uninstalled headers in src/ with
a _p at the end of the name, to make it clear they are not part of the
API. None of the headers in KImageFormats are installed, so it is not
really necessary to follow this convention, but we follow it anyway for
the benefit of both humans and tools (like kapidox).

src/imageformats/rgb_p.h

@@ -0,0 +1,38 @@
+// kimgio module for SGI images
+//
+// Copyright (C) 2004  Melchior FRANZ  <mfranz@kde.org>
+//
+// This program is free software; you can redistribute it and/or
+// modify it under the terms of the Lesser GNU General Public License as
+// published by the Free Software Foundation; either version 2 of the
+// License, or (at your option) any later version.
+
+#ifndef KIMG_RGB_H
+#define KIMG_RGB_H
+
+#include <QImageIOPlugin>
+
+class RGBHandler : public QImageIOHandler
+{
+public:
+    RGBHandler();
+
+    virtual bool canRead() const;
+    virtual bool read(QImage *image);
+    virtual bool write(const QImage &image);
+
+    static bool canRead(QIODevice *device);
+};
+
+class RGBPlugin : public QImageIOPlugin
+{
+    Q_OBJECT
+    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QImageIOHandlerFactoryInterface" FILE "rgb.json")
+
+public:
+    virtual Capabilities capabilities(QIODevice *device, const QByteArray &format) const;
+    virtual QImageIOHandler *create(QIODevice *device, const QByteArray &format = QByteArray()) const;
+};
+
+#endif // KIMG_RGB_H
+