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.