Add shortcut to toggle visibility of all overlay layers

[Nekzuris]

Description

Similarly to the Alt + W shortcut that hides the data layer, it would be nice to have a shortcut to turn on/off the visibility of the overlay layers.
I suggest Alt + Q but be careful with Alt shortcuts on Firefox because the Alt key also reveal the top menu bar in some situations. Might be better to move these 2 shortcuts to Shift.
This was briefly discussed here for the Strava iD extension julcnx/id-strava-heatmap-extension#12 (comment) .

[tyrasd]

Shift + <something> would also fit the relatively new Shift + P shortcut to toggle street-level photo layers.