Consolidate all convert_* functions into libi3.

Some minor fixes along the way as well. Very minor stuff, unlikely
to ever be visible to the user.

i3-input/i3-input.h

@@ -14,7 +14,4 @@ while (0)
 
 extern xcb_window_t root;
 
-char *convert_ucs_to_utf8(char *input);
-char *convert_utf8_to_ucs2(char *input, int *real_strlen);
-
 #endif