A few small fixes

PlainTasksOpenLinkCommand:
✔ fix regex to prevent a closing (back)slash being considered as part of
filename — relative paths are not allowed anymore, but filenames with
whitespaces works properly and it's possible to add any additional text
after (back)slash;
✔ fix the issue when instead of switching to opened file, command spawns
quick panel with many duplicates.
tmLanguage:
✔ remove attempt to match date in done/cancelled tasks when tags are
absent — it ain’t work anyway.
Tutorial.todo:
✔ update info about file links;
✔ replace few regular whitespaces with nonbreaking ones — prevent
undesirable highlighting for @.