qt5: package png image files of QML components too

Fixes the following packaging warnings with qtdeclarative 5.1.0:

WARNING: QA Issue: qtdeclarative: Files/directories were installed but not shipped
  /usr/lib/qt5/qml/QtQuick/Dialogs/images
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/sunken_frame.png
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/window_border.png
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/slider_handle.png
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/folder.png
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/up.png
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/checkers.png
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/copy.png
  /usr/lib/qt5/qml/QtQuick/Dialogs/images/crosshairs.png

Signed-off-by: Simon Busch <morphis@gravedo.de>