Sketcher: Multi-drag and special drag

[jnxd]

We would like to know there is interest in dragging multiple sketch elements at a time.

This topic stems from my recent PR #7110, which changes the drag behavior of B-splines such that only a part between knots is moved. This change raises a question of what to do when the one wants to move multiple segments. One solution is to be able to select the poles of the curves and then move them accordingly.

Alternatively, a modifier key could be used while dragging to say we want to move the whole curve preserving shape. Other modifier keys can be used to move curves with some restrictions, like along 45 degree increments similar to inkscape and other tools.

Apparently the block select and drag feature existed previously, but was removed, presumably during a refactoring.

@abdullah was there anything specific you wanted to add?