utility for generating icon themes and libXcursor cursor themes

 The inputs to icon-slicer are conceptually:
 .
  - A set of multi-layer images, one for each size
  - Am XML theme description file
 .
 Each image contains all the cursors arranged in a grid; For cursors the
 layers are:
 .
  - A layer with a dot for the hotspot of each cursor
  - The main image or first animation frame for multi-frame animated
    cursors
  - The second animation frame for multi-frame animated cursors
  - ...
 .
 For icons, the layers are:
 .
  - A layer with the images
  - An optional layer with attachment points for emblems
  - An optional layer with boxes for embedding text into icons
 .
 In practice, since loading of multilayer images is not supported by
 standard image libraries, each layer is input as a separate image file.
 .
 The theme description file contains, among other things, information
 about the source images to read, the location of each named cursor or
 icon within the grid, and a set of aliases from names to other names.