Implement vim-like marks

Commands are 'mark' and 'goto'. Both can be used either directly,
like 'mark a' and 'goto a', or interactively (just 'mark'). For
interactive mode, i3-input must be installed and in your PATH.

i3-input/i3-input.h

@@ -6,6 +6,7 @@
 #define die(...) errx(EXIT_FAILURE, __VA_ARGS__);
 
 char *convert_ucs_to_utf8(char *input);
+char *convert_utf8_to_ucs2(char *input, int *real_strlen);
 uint32_t get_colorpixel(xcb_connection_t *conn, char *hex);
 uint32_t get_mode_switch_mask(xcb_connection_t *conn);
 int connect_ipc(char *socket_path);