Use QImage.bits instead of QImage.constBits to ease backporting

Image.constBits was introduced in Qt 4.7.  Should be ok here to use
QImage.bits which allows linking against the Qt in Squeeze.
Merge pull request #2289 from laanwj/201301_qimage_bits

Use QImage.bits instead of QImage.constBits to ease backporting
Add script to clean up datadirs
Some improvements to the release notes
Merge pull request #2297 from sipa/notes

Some improvements to the release notes
Merge pull request #2295 from sipa/tidydir

Add script to clean up datadirs