Now the option is always enabled, but the user can disable by holding
Alt modifier. The new behavior is the following:

No Alt pressed:

1) Scale -> scales relative to the *opposite corner*
2) Rotate -> rotates around the anchor point
3) Shear -> shear around the anchor point

Alt is pressed:

1) Scale -> scales relatinve to the *anchor point*
2) Rotate -> rotates around the center
3) Shear -> shear around the center

Tool options controls:

Always transform around the anchor point. If the user wants to
transform around center, he should just move the anchor to the
center (by clicking on the corresponding button).

Problems:

The only problem of this approach is that one cannot combine Alt modifier
with Ctrl (perspective) and Shift (keep aspect ratio) modifiers. I don't
know if it is a critical problem or nor.