Modified zoom code:

Modified zoom code:
- Now keeps track of zoom levels, rather than screen scale
- Calculates home scale from screen DPI
- View size changes are tracked, and if selected zoom_to_fit is maintained.
- New windows default to zoom_to_fit.
- Tracks changes in screen, so that new home scale can be updated

git-svn-id: https://glabels.svn.sourceforge.net/svnroot/glabels/trunk@352 f5e0f49d-192f-0410-a22d-a8d8700d0965