Cursor manipulation not intuitive

[tomverbeure]

The current behavior of cursors has a bunch of issues:

• when you have 2 vertical cursors on the screen and you click on a waveform panel while not being near any of the cursors, both cursors are moved to the new location, instead only moving the nearest one, thus potentially losing one of the positions that you've carefully placed. The expected behavior is either that nothing happens (because you're not close to any cursor) or that you only move the closest one. However, with the second option, an additional complication pops up when you have vertical and horizontal cursors (what if you only want to move one but not the other), so the first option is preferred.
• when you have both vertical and horizontal cursors and you try to move a horizontal way, the vertical cursor will also always move as well. This is super annoying and not in line with how cursors behave on other oscilloscopes.

[azonenberg]

This probably makes sense to split into two tickets.

The intent is that click and drag when not near a cursor places two cursors for making measurements. I can see this as a potential enhancement for some people while others would prefer the current way, which seems like a good use for a preference setting.

Collision detection for one cursor not being aware of drag state of the other is indisputably a bug and needs to be fixed.

[azonenberg]

Filed #965 for the collision detection issue, let's keep 963 for the "some people find the intended can-drop-new-cursor-pair-anywhere behavior unintuitive" side.