Add .xpm support for <icon> via libXpm.

* Adds a new flag `with_xpm` to enable compilation with xpm support
* Adds a module `XPMFile` only exporting `readXPMFile` which almost mirrors `Graphics.X11.Xlib.Misc.readBitmapFile`
* During `loadBitmap` a file is first tried with `readBitmapFile` and if it fails with `readXPMFile`

The overall gain is a support for multicolored images.
perhaps we could add also an example of using an xpm to one or two of the spots in the readme where icon support is mentioned, or just mention there in passing that xpm can also be supported