Improve handling of file names in the VCS output pane

The VCS output pane already allows to right-click on filenames and to open them in an editor.

I have two extensions for that in mind:

1. Unfortunately, there is no visual feedback that the filename can be clicked. Most people will simply not know. If they were e.g. links, that would be obvious.

2. Depending on the state of the file, other context menu actions might also make sense:

• File is changed: add actions to show a diff and to revert it
• File is deleted: add action to restore it
• File is untracked: add action to add it to the repo