Misc libkis/python-related fixes and small additions

I've been writing a krita python plugin for my own use, and I've fixed libkis bugs I've come across in the process. There's also two tiny functions added that I needed.

The View::setDocument commit is pretty related to https://bugs.kde.org/show_bug.cgi?id=393464 to the point it could close it, if you ignore actually having an empty view being impossible.