Selection in diff to copy lines to clipboard

[extrawurst]

I regularly want to copy stuff out of diffs.

This requires supporting selecting multiple lines in the diff view.

Then we can add a new command to copy the selected block into the clipboard.

This selection function would also be the foundation for per line staging (#59)

[extrawurst]

I see this as a two step process:

1. building the selection functionality into the diff view component to not only have a scroll position but a selection start position and figure out if we can use shift+arrows for this and visualize it in the draw method. if the shift key is not pressed while scrollen we can simply do selection_start=selection

1.b) hopefully we can use the Diff object the view is already saving to pull out the content of the selected lines otherwise we might need a new feature inside asyncgit (I would argue we dont)

2. look into using https://crates.io/crates/clipboard to put the selected lines into os clipboard