Small features and improvements

[harshad1]

This PR includes a bunch of small features and improvements:

1. Surrounding text with delimiters (for example markdown bold) happens in 1 step. This makes undo easier
2. Removing surrounding delimiters is also made slightly better (select text and press corresponding action)
3. Long press bold, italic, strikethrough to make the whole selected line strikethrough etc
   • Only for markdown
4. The up and down arrow buttons now repeat. Can hold them to move selected line(s) up and down easily
5. There is an option in the file menu to show the currently open document in the file explorer
6. Can add multiple files to favourites in multi select mode
7. Added option to share path of currently open file
8. The select snippet directory option will now show the selected path
9. Reorganized how actions are specified in derived action button classes. Common actions are moved to ActionButtonBase.

[harshad1]

This is now ready

[gsantner]

Sorting by filesize seems to be broken currently. Guess due the sort-key changes. Can you reproduce that?

[harshad1]

@gsantner I completely missed your comment re file sorting. This is fixed. Added zero padding to solve it.

[harshad1]

@gsantner This is ready to merge. Please let me know if you need changes.

I will try and keep these smaller and split them up in the future :)

[gsantner]

Thanks!