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/data.h

@@ -230,12 +230,29 @@ struct Workspace {
  *
  */
 struct Binding {
+        /** Symbol the user specified in configfile, if any. This needs to be
+         * stored with the binding to be able to re-convert it into a keycode
+         * if the keyboard mapping changes (using Xmodmap for example) */
+        char *symbol;
+
+        /** Only in use if symbol != NULL. Gets set to the value to which the
+         * symbol got translated when binding. Useful for unbinding and
+         * checking which binding was used when a key press event comes in.
+         *
+         * This is an array of number_keycodes size. */
+        xcb_keycode_t *translated_to;
+
+        uint32_t number_keycodes;
+
         /** Keycode to bind */
         uint32_t keycode;
+
         /** Bitmask consisting of BIND_MOD_1, BIND_MODE_SWITCH, … */
         uint32_t mods;
+
         /** Command, like in command mode */
         char *command;
+
         TAILQ_ENTRY(Binding) bindings;
 };