xep-0308: Implement /correct to correct the last send message

So far the correction is sent. But the UI in Profanity itself is not
updated.

Also autocompletion for `/correct` with the last sent message is
missing.

src/command/cmd_defs.c

@@ -2367,6 +2367,7 @@ static struct cmd_t command_defs[] =
         CMD_NOSUBFUNCS
         CMD_MAINFUNC(cmd_correction)
         CMD_TAGS(
+            CMD_TAG_UI,
             CMD_TAG_CHAT,
             CMD_TAG_GROUPCHAT)
         CMD_SYN(
@@ -2379,6 +2380,22 @@ static struct cmd_t command_defs[] =
             { "char",    "Set character that will prefix corrected messages. Default: +"})
         CMD_NOEXAMPLES
     },
+
+    { "/correct",
+        parse_args, 1, 1, NULL,
+        CMD_NOSUBFUNCS
+        CMD_MAINFUNC(cmd_correct)
+        CMD_TAGS(
+            CMD_TAG_CHAT,
+            CMD_TAG_GROUPCHAT)
+        CMD_SYN(
+            "/correct <message>")
+        CMD_DESC(
+            "Correct and resend the last message (XEP-0308).")
+        CMD_ARGS(
+            { "<message>",    "The corrected message."})
+        CMD_NOEXAMPLES
+    },
 };
 
 static GHashTable *search_index;