Add a toggle for screen shake effects

[v1993]

Fixes #3609.

Marking as draft since I've realized it might be a good idea to make it tri-state instead:

• Full (default): both one-time shakes (e.g. from bombs) and "earthquakes" (persistent shaking) enabled
• Reduced: only one-time shakes enabled
• Off: both one-time shakes and "earthquakes" disabled

Should I go for that? If so, should I bother with an enumeration (like video subsystem selection) or would a simple int (like for vsync) do?

[swagtoy]

Should I go for that?

Yes

should I bother with an enumeration

For tri-state, yes. It does no harm

[v1993]

Changed to off/reduced/full selection as proposed, seems to work well in testing. Using an enum means that there are three formats to convert between (selection number <-> enum <-> string), which is a bit unwieldy, but ultimately works okay.

[swagtoy]

I'll review tomorrow, ping if I forget. Thank you very much for your work!

[v1993]

@swagtoy Ping! Also, a look at #3540 would be appreciated as well.

[swagtoy]

I'm really sorry. will get back to this soon

[MatusGuy]

Could this interfere with string freeze?