Cleanup PanZoom.vue

[saraedum]

The PanZoom component has some issues at the moment. Namely,

• the semantics of the two-way binding of the focused value are not completely clear. Setting a focus, reports back a focused box in the viewport coordinate system. When the ideal coordinate system is modified, we try to refocus on that focused box which might not be possible, see try/catch in PanZoom.vue. To make that refocusing more reasonable, we need some hacks in Viewer.vue.
• the responsibilities of mounted/beforeDestroy and initialize/cleanup are not really clear.
• the automatic resizing might be done in a better way. Again, the refocusing does not always work out great.

Probably, we should limit the capabilities of PanZoom quite a bit. E.g., by reworking/removing the two-way binding and by disallowing a change of coordinate system.

In any case, this needs lots of testing as this component breaks very frequently.