better visuals for deleted files

[FloppyDisco]

deleted files are not very well indicated in Quick Stage

in the native SCM view they are 'strike-through' and when you 'Open' them they show the file with a tab title of "Deleted"

this behavior should be mimicked as best as possible.

currently no preview is being shown at all.

• show a preview
• visually change the menuItem to indicate the file is deleted

[FloppyDisco]

the native view opens the file in an editor with a "deleted" subtitle in red. figure out how this is done

[FloppyDisco]

attempting to open a deleted file in DIFF is actually causing a silent error. this needs to be handled when the file is attempted to be opened.