[amiwm] Add the basic battery info printing bits

* add a batteryinfo config option to enable whether or not to display
  the current battery info string

* populate that battery info string whenever we receive a battery
  update from the module

* document it all

This is very specific to a battery thing rather than a generic menu
widget thing, but I think I'll go and twiddle with this stuff a bit
more first and then work on a more generic "menu widget" thing later.

It'd be nice to have "menu bar things" like battery and the date/time
field on a linked list of "things".

There's also some bugs - notably, rendering artifacts when the
string length shrinks, and it only updating on the 'current' menu bar
(when you have multiple desktops visible at once.)  Those also
should be fixed for completeness. :-)

rc.c

@@ -105,6 +105,7 @@ struct keyword { char *name; int token; } keywords[] = {
   { "barblockpen", T_BARBLOCKPEN },
   { "bardetailpen", T_BARDETAILPEN },
   { "bartrimpen", T_BARTRIMPEN },
+  { "batteryinfo", BATTERYINFO },
   { "blockpen", T_BLOCKPEN },
   { "both", BOTH },
   { "bottom", BOTTOM },