Add favourite indicator

[delphidabbler]

Later Version

Add indicator to snippet detail pane that shows if snippet is a favourite.

Make indicator clickable to toggle state.

Possibly delete toolbar button since that will just duplicate clickable indicator

[delphidabbler]

This is a low priority request, since there's already a way of making a snippet a favourite.

[delphidabbler]

Going to defer this to a major version. CodeSnip vault has made some changes to the details pane that make implementing this more straightforward.