Batch export operations

[krupkat]

Base: Implement "Export all" action
Stretch goal: Implement "Export selected" action

Starting point:
Code location

[krupkat]

Some more though needs to go into how panorama state is being handled.

Currently the crop / autofill results are being kept in memory only for the selected panorama. This is because the full resolution panoramas can quickly go into GBs of memory, so it isn't feasible to keep all of them.

We could save only the crop / autofill options per each panorama for lower memory requirements, however the stitch algorithm isn't deterministic, so the result could be quite surprising (especially for crop).

This should be solved by saving the camera parameters after the first successful stitch and then reapplying them in the export/stitch code (setTransform).

[krupkat]

Test of the feature: https://github.com/krupkat/xpano/tree/feature-batch

The approach of saving + restoring camera parameters works well, implementing this will mean that Xpano won't need to recompute the camera parameters when switching between panos.

Eventually export + import of the camera parameters can be implemented.