Improved PNG import

I've improved the PNG importer. First of all, I've moved the importer code to import_export.py. Then, I've added support for loading non-indexed PNG files. The PNG import now works as follows:

* If the image is indexed, import it as usual.
* Else if there already is a palette, use Qt's builtin functionality to perform a closest color conversion.
* Else, try to sniff a color palette from the image. This will only work if the image contains less than 256 colors.
* If the image contains more colors, open a dialog where the user can select an algorithm to be used when indexing. Then index with Qt's builtin functionality.