Add shortcut to toggle overlay layer visibility

[zandonella]

Added a shortcut to toggle the visibility of all overlay layers to Shift + O. This follows the Shift + <key> pattern set by newer shortcuts (e.g., Shift + P) and, as far as I'm aware, does not conflict with any browser shortcuts. I chose O as the key as it is adjacent to the existing Shift + P shortcut on QWERTY keyboards, and is an intuitive letter for "Overlays" to be on.

I used essentially the same logic as toggleStreetSide for toggleAllOverlays with some modifications due to the differences in layer structure. Because the list of selectable overlays has a variable number of overlays, I also had to modify updateLayerSelections to stop them from unchecking on rerender.

Closes #10972

Keybind in keyboard shortcuts:

Visibility on (default):

Visibility toggled:

[tyrasd]

FWIW, the iD Strava Heatmap plugin adds Shift+Q as a keyboard shortcut for toggling the overlay layers. That has the small benefit of being usable with the same hand on the keyboard as one would use to toggle the OSM data layer (Alt+W). On the other hand, Shift+O can be easier to remember (O = overlays). 🤔