Port to new selection and document API. KolourPaint finally compiles after days.

In summary, the API changes mean:

* Selections now finally have a proper class hierarchy
- It is now actually possible to maintain the selection code since it's all been split into separate files
- You can't accidently invoke text operations on image selections nor image operations on text selections anymore

* Text selections with no text are treated now as selection borders, so creating text boxes should not mutate the document, until text is typed

* Bugs have been identified even in KolourPaint/KDE3 (see added TODOs)

* Regressions
- Text selections are unlikely to work
- As we can't create an instance of the abstract kpAbstractSelection, we use pointers more often and so there are probably more memory leaks
- There were lots of change so I've probably broken lots of other things.