Layer selection history

[Keavon]

The Forward/Backward mouse navigation buttons should enable the user to move through selection history (similar to VS Code's text selection), selecting different sets of layers as they were throughout the past. Layers which don't exist anymore, but were selected in the past, presumably would just not end up being selected. (And if zero of layers that were selected during a selection history state in the past end up existing now, filter that history state out.)

A note about capturing the Forward/Backward mouse buttons.