Refactor kgraphicswebslice

With this larger update, we tie down the whole setup process, and waste a lot less cycles resizing.

In particular:
- zoom method: zooms to specific slice
- less unnecessary resizing (still room for improvement)
- make switching between preview and slice mode work
- fix slice geometry for previews and slice modes
- cache geometries: faster and less dicking around with zoomFactor
- Make applet freely resizable: no fixed aspect mode anymore
- Resizing is also a lot faster now

Cleanup pending.