Redrawing an icon every time the cursor moves between items costs a lot
of time.  So now each icon is drawn once, and s copied when needed.

Changing config.cursx and config.cursy to config.posx and config.posy.
Adding the parseposition() function.

Widths and x positions no more depend on item->h.

• Adding support for gaps (#3)
• Adding more comments and categorizing function declarations.
• Adding support for EWMH properties (xmenu look is now consistent
  in some window compositors).
• Changing how configuration is done (now all configuration belongs
  to the config structure).