OCGV: Add UI for select-all/select-none

[tig]

When dealing with large lists and -OutputMode Multiple, the ability to select all items, or deselect all is desired.

Proposal: When -OutputMode: Multiple is provided, add two shortcut keys to `StatusBar1:

On Windows/Linux ^A would be CTRL-A. On Mac it would be Command-A.

I chose ^A because that is a common idiom in GUIs for Select-All. In Windows, for example, multi-select listboxes handle CTRL-A automatically. There is no standard (that I could find for select-none) but in some examples I've found folks use CTRL-SHIFT-A so I went with that.