feat: Sanitize illegal XML characters from outgoing messages

Filter out control characters (U+0000 to U+001F) from outgoing
messages, as they are illegal in XML 1.0 (except for \t, \n, and \r).
This prevents XMPP servers from closing the connection when such
characters are accidentally or intentionally included in a message.

Fixes: https://github.com/profanity-im/profanity/issues/1437

src/common.h

@@ -159,6 +159,7 @@ gboolean copy_file(const char* const src, const char* const target, const gboole
 char* str_replace(const char* string, const char* substr, const char* replacement);
 gboolean strtoi_range(const char* str, int* saveptr, int min, int max, char** err_msg);
 int utf8_display_len(const char* const str);
+gchar* str_xml_sanitize(const char* const str);
 
 gboolean string_matches_one_of(const char* what, const char* is, gboolean is_can_be_null, const char* first, ...) __attribute__((sentinel));
 gboolean valid_tls_policy_option(const char* is);