Make text tool work 100% - port transparent text painting to Qt4. It seems that painting text in Qt::color0 works in Qt4 so no need for the elaborate hacks we had before with contrasting colors. Now it's fast enough so that we don't need a text image cache either.

Later: Unfortunately, I did this wrong and a transparent foreground color "sets" pixels instead of painting them, thus acting differently to transparent background pixels. Will need to revert bits and pieces of this commit and adjust some things.

We rename kpTempImage::mayChangeDocumentMask() to the more accurate paintMayAddMask() and also add one for selections (but this latter change will be reverted due to second paragraph).