Share the whole rasterize logic of the text, link and vector layer into
an interface. I didn't write it as an abstract parent class, because we
might have more rasterizable items in the future, which may not be
layers (e.g. there were discussions of vector masks).
This patch improves vector layer UX based on feedback.
In summary:
* Makes vector layer editable from the Path tool
* Adds initial PDB for creating vector layers in scripts
* Size vector layers to the path size, rather than image
* Transform tools utilize the path for resizing
* Path tool automatically selects vector layer path
