Put getting mentions in own function

So we can use it somewhere else too.

Regards https://github.com/profanity-im/profanity/issues/1261

src/common.c

@@ -510,3 +510,18 @@ get_random_string(int length)
 
     return rand;
 }
+
+GSList*
+get_mentions(gboolean whole_word, gboolean case_sensitive, const char *const message, const char *const nick)
+{
+    GSList *mentions = NULL;
+    char *message_search = case_sensitive ? strdup(message) : g_utf8_strdown(message, -1);
+    char *mynick_search = case_sensitive ? strdup(nick) : g_utf8_strdown(nick, -1);
+
+    mentions = prof_occurrences(mynick_search, message_search, 0, whole_word, &mentions);
+
+    g_free(message_search);
+    g_free(mynick_search);
+
+    return mentions;
+}