fixes bug 4495
- add missing edge faces
- implement a proper rotation recipe -> rotate towards the nearest possible state
- add preferences settings to turn off this new rotation behavior
- get rid of transparent cube faces to avoid wrong clicks, this way also remove the backfaces
- to make the edge faces fit, use triangles for the corners
- expose cube size setting to preferences dialog
- add new button to view backside
- get rid of bitmaps because as reported in https://forum.freecadweb.org/viewtopic.php?f=3&t=52118&start=20#p481781
the x y z labels are not scaled on HiDPI screens
moreover they are often "in they way" when you want to click on a corner and in principle they don't provide more info since we already have the CS with the axis labels
- also fix an arithmetic overflow compiler warning
- also rename 2 variables
