Add support for UTF-8 key bindings

A new function keys_wgetch() reads full UTF-8 characters instead of
single ASCII characters only.

Key bindings for regular ASCII characters are stored in a hash map while
the actions of keys with higher code points are stored in a linked list
for space efficiency.

The key serialization methods are updated to handle UTF-8 characters as
well; extended UTF-8 characters (characters not in the ASCII range) are
serialized by using the hexadecimal representation of the corresponding
code points (e.g. "U+00E4").

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/custom.c

@@ -972,10 +972,10 @@ void custom_keys_config(void)
 					  (col - WINCOL) / 2,
 					  _("Press the key you want to assign to:"),
 					  keys_get_label(selrow), 0);
-				ch = wgetch(grabwin);
+				ch = keys_wgetch(grabwin);
 
 				/* First check if this key would be recognized by calcurse. */
-				if (keys_str2int(keys_int2str(ch)) == -1) {
+				if (ch < 0) {
 					not_recognized = 1;
 					WARN_MSG(_("This key is not yet recognized by calcurse, "
 						  "please choose another one."));