Potential usability improvements

While going through a small dataset and annotating a couple of membranes, I encountered some minor issues. Fixing these could potentially improve usability of the software.

I'm collecting them here:

- [x] Orientation deletion: After deleting points, corresponding orientation arrows are still in place (however, they are removed when clicking a new point). Can we delete points with orientation arrows together? (#36 )
- [ ] Moving points: When adjusting already placed points, they are not necessarily placed on the membrane surface. Would it make sense to also place them on the surface?
- [x] when moving points, orientation arrows stay in the original position -- would be nice if they also moved with the points (#36 )
- [ ] When shifting the view (i.e. hold shift + drag), unwanted points are placed on the surface as well (because of the click). Can we deactivate point placement when holding shift?
- [x] "Enable Picking" only instantiates new points / orientations the first time. If I delete them and want to start clicking again, it does not work (#37)
- [ ] Orientation arrow sometimes disappears within the membrane (in case of curvature I guess) —> maybe we can place it a bit further up (along the normal)?
- [x] Shading should be „none“ by default. This improves appearance of the densities a lot (#37)
- [x] For new picks: rotation slider should be reset to default. This would make it easier to interpret the slider value (#42 )

I didn't have the time yet to look into them, and don't know how easy each of them is, but happy to do it when I'm back from vacation.

Start by reproducing the unchecked point-placement and orientation-arrow behaviors described in the issue while annotating membranes. Review the remaining items separately, and consider the work done when the specified interactions no longer place unintended points, keep arrows aligned, and preserve visible orientations.

Written by the indexing model from the issue text.