Implement support for using key symbols in configuration file

Use "bindsym" instead of "bind". You have to use the names of keys
as in xmodmap. To get a list of currently bounud symbols, use
xmodmap -pke

Technical quirk: Xlib generated MappingNotify events upon
XkbMapNotify events (from XKB, as the name says). XCB does not yet
have support for XKB, thus we need to select and handle the event
by ourself. Hopefully, this will change in the future.

include/config.h

@@ -79,6 +79,18 @@ struct Config {
  *
  */
 void load_configuration(xcb_connection_t *conn, const char *override_configfile, bool reload);
+
+/**
+ * Ungrabs all keys, to be called before re-grabbing the keys because of a
+ * mapping_notify event or a configuration file reload
+ *
+ */
+void ungrab_all_keys(xcb_connection_t *conn);
+
+/**
+ * Grab the bound keys (tell X to send us keypress events for those keycodes)
+ *
+ */
 void grab_all_keys(xcb_connection_t *conn);
 
 #endif