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_completion

@@ -11,18 +11,14 @@ _todo()
     local -r OPTS="-@ -@@ -+ -++ -d -f -h -p -P -PP -a -n -t -v -vv -V -x"
     local -r COMMANDS="\
         add a addto addm append app archive command del  \
-        rm depri dp do help list ls listall lsa listcon  \
+        rm depri dp do help list ls listaddons listall lsa listcon  \
         lsc listfile lf listpri lsp listproj lsprj move \
         mv prepend prep pri p replace report shorthelp"
 
-    # Add custom commands from add-ons, if installed.
-    # TODO: Filter for executable flag of files found in $TODO_ACTIONS_DIR.
-    local allCommands="$COMMANDS $('ls' "${TODO_ACTIONS_DIR:-$HOME/.todo.actions.d}/" 2>/dev/null)"
-
     local _todo_sh=${_todo_sh:-todo.sh}
     local completions
     if [ $COMP_CWORD -eq 1 ]; then
-        completions="$allCommands $OPTS"
+        completions="$COMMANDS $(eval TODOTXT_VERBOSE=0 $_todo_sh command listaddons) $OPTS"
     elif [[ $COMP_CWORD -gt 2 && ( \
         "${COMP_WORDS[COMP_CWORD-2]}" =~ ^(move|mv)$ || \
         "${COMP_WORDS[COMP_CWORD-3]}" =~ ^(move|mv)$ ) ]]; then
@@ -34,7 +30,7 @@ _todo()
                 completions=$COMMANDS;;
             addto|listfile|lf)
                 completions=$(eval TODOTXT_VERBOSE=0 $_todo_sh command listfile);;
-            -*) completions="$allCommands $OPTS";;
+            -*) completions="$COMMANDS $(eval TODOTXT_VERBOSE=0 $_todo_sh command listaddons) $OPTS";;
             *)  case "$cur" in
                     +*) completions=$(eval TODOTXT_VERBOSE=0 $_todo_sh command listproj)
                         COMPREPLY=( $( compgen -W "$completions" -- $cur ))