Rename rotation center to transformation center

Summary:

• Rotation Center affects all the three transformations available: Rotation, Skewing, and Scaling. So, it should better be called Transformation Center.
• This Rotation/Transformation Center should be shown in Scale mode too - it will improve discoverability of this feature.

Steps to try:

• open Inkscape
• Draw a rectangle and choose Select tool
• Try all the aforementioned 3 transformations around rotation/transformation center, as well as around opposite side (use Shift modifier key)
• Try to tinker with the rotation/transformation center position for all these transformations separately

What happened?

You can't change the rotation/transformation center for scaling in the scale mode. It can be repositioned only in Rotate/Skew mode.