start coordinate system builders from the center

[maymage]

Hi all,

when creating a 2d- or 3d-coordinate system, it would be more reasonable to position the center of the coordinate system at the first touching point of the pen. After that, height and width (and thereby depth), can be determined by moving the pen away from the center point.

In fact, when I draw a coordinate system, I surely know, where to put the center with the desired hight and width of the coordinate system in mind - not the other way round.

That's also easy to explain: for a given center, to estimate hight and width, is a one-to-two map, which is much more easy than a two-to-one map, i.e. given hight and width to estimate the resulting center.

[PhilDevProg]

Maybe have both options? Because if you start at the center and then realize, that you don't have enough space above the coordinate system and you have to select it, move it and then resize it (which doesn't work well for specific lines of it atm, See #623). That can't happen with the current method.