Display i3-nagbar when commands lead to an error

e.g. pressing Mod1+x when having the following in your configfile:

    bindsym Mod1+x some invalid command

will lead to an i3-nagbar instance popping up, offering you to view the
error log (which will contain parser errors from this commit on).

include/key_press.h

@@ -0,0 +1,32 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009-2012 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * key_press.c: key press handler
+ *
+ */
+#ifndef _KEY_PRESS_H
+#define _KEY_PRESS_H
+
+/**
+ * There was a key press. We compare this key code with our bindings table and pass
+ * the bound action to parse_command().
+ *
+ */
+void handle_key_press(xcb_key_press_event_t *event);
+
+/**
+ * Kills the commanderror i3-nagbar process, if any.
+ *
+ * Called when reloading/restarting, since the user probably fixed his wrong
+ * keybindings.
+ *
+ * If wait_for_it is set (restarting), this function will waitpid(), otherwise,
+ * ev is assumed to handle it (reloading).
+ *
+ */
+void kill_commanderror_nagbar(bool wait_for_it);
+
+#endif