Issue #526 - Add Import/Export for Windows .CUR files

(Requesting feedback - this feature works on my build, but I'm not sure if I've structured the code properly)

The CUR format for Windows Cursors is practically identical to the ICO format (https://docs.fileformat.com/image/ico/). The main difference is the planes and bpp fields are interpreted as X/Y coordinates for the cursor hotspot if the header's resource type is set to 2 instead of 1.

This commit attempts to resolve #526 (closed) and provides support for importing .CUR files via the file-ico plugin. Cursor hotspot input, retention, and saving are modeled after similar features in file-xbm. When exporting via dialog, entry fields for the X/Y coordinates are conditionally added to the normal ICO display: