Added UI for converting a Pixel Selection into a Vector Selection

Now you can click the menu and your selection will be converted into
a vector one. After this action you can edit it with a shape editing
tooleasily: rotate, scale, edid vertices.

DRAWBACK: you lose information about semi-transparent pixels during
          this conversion!

The backward conversion (vector->pixel) happens automatically when you
start painting on a vector selection.

CCMAIL:kimageshop@kde.org