Our build files were relying 'sysroot' to find gexiv2.h but this is
not possible with Apple Clang om which sysroot points to macOS SDK.
So, exotic environments like Homebrew were failing. Let's fix this.
We now have a proper PROP_VECTOR_LAYER property in the XCF which will
store the various data relevant to vector layers.
This also fixes saving this new type of layers at all, since it was
broken in MR !773.
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
