[amiwm] MVP for initial battery stuff

Totally not ready for prod, but at least I'm making progress
figuring out how to add extra bits to this thing.

* add a second widget to the menu bar, next to the title bar
* add a new module command to update the battery information
* add a freebsd specific Battery module, not linked into the
  build right now.
* amiwm will print out whenever we get battery information from the
  module.

Right now I'm trying to figure out how to get some kind of periodic
background event into the module main loop.  Launcher does it
using cx_broker(), but I dunno if I can adapt that pattern here.

libami/libami.h

@@ -389,6 +389,9 @@ extern Pixmap md_image_to_pixmap(Window, unsigned long, struct Image *,
 				 int, int, struct ColorStore *);
 extern char *get_current_icondir(void);
 
+/* mdbattery.c */
+extern void md_update_battery(int pct, int time, int ac);
+
 /* mdwindow.c */
 extern int md_set_appwindow(Window);