ENH: Better completion for custom add-ons via new listaddons command.

The todo_completion had the problem that it didn't consider the todo.cfg itself for the location of TODO_ACTIONS_DIR, it just tried the default location or a globally exported config value. With the injection of custom configuration now in place, we can actually delegate the listing to todo.sh itself.
The added built-in "listaddons" command is used for that; it may also be helpful for troubleshooting or to find out about available add-ons. (But the help / shorthelp commands offer more information.)

Additionally, completion is now more precise; only executable actions and no subdirs are listed now; this is also covered by the enhanced test.

The last bonus: The custom add-on actions are now only determined when there's actually completion of commands.

todo.sh

@@ -57,6 +57,7 @@ shorthelp()
 		    help
 		    list|ls [TERM...]
 		    listall|lsa [TERM...]
+		    listaddons
 		    listcon|lsc
 		    listfile|lf [SRC [TERM...]]
 		    listpri|lsp [PRIORITIES] [TERM...]
@@ -219,6 +220,9 @@ help()
 		      TERM specified, lists entire todo.txt AND done.txt
 		      concatenated and sorted.
 
+		    listaddons
+		      Lists all added and overridden actions in the actions directory.
+
 		    listcon
 		    lsc
 		      Lists all the task contexts that start with the @ sign in todo.txt.
@@ -1300,6 +1304,18 @@ note: PRIORITY must be anywhere from A to Z."
     fi
     ;;
 
+"listaddons" )
+    if [ -d "$TODO_ACTIONS_DIR" ]; then
+        cd "$TODO_ACTIONS_DIR" || exit $?
+        for action in *
+        do
+            if [ -f "$action" -a -x "$action" ]; then
+                echo "$action"
+            fi
+        done
+    fi
+    ;;
+
 * )
     usage;;
 esac