Add import/export support for Clipping Paths in PSD

This patch attempts to add import/export support for clipping paths (Resource 2999) in PSD documents. Clipping paths allow you to choose an existing vector path that (optionally) clips out backgrounds when used to programs like Scribus and InDesign. The goal is to enable this workflow for Gimp users.

The header format is simple:

[2 bytes for total length] [1 byte for string length] [Pascal String for Path name] [4 bytes for fixed number "flatness" value] [2 bytes for fill rule (ignored by Photoshop]

The patch stores the path name and flatness float value in parasites on load. On export, a new GUI element loads the information from the parasite and then lets the user change either value. It is then optionally saves the clipping path resource to file.

The output of this patch can be tested without Photoshop by checking the metadata with ImageMagick. It can also be checked by adding the exported .psd into a Scribus image frame and then choosing "Extended Image Properties". Under the "Paths" tab, the path considered the clipping path will be highlighted.

Feedback appreciated! (@Wormnest, since you're familiar with this format I'd be interested in your thoughts when/if you have time)

(This is also a precursor to completing !756 (closed), as the load/export functions will be centralized in PSD and then accessed in other plug-ins like TIFF and JPEG)