Add a key for the retaliate action

[wiki-me]

Is your feature request related to a problem? Please describe.

Lets say i am doing something (say looking at information of potential target nations), someone attacks me. i have to move my mouse to the retaliate button which takes a while.

Describe the solution you'd like

allow retaliating by pressing a button, i suggest the detault will "r".

Describe alternatives you've considered

no real alternative

Additional context

A requested this on this pull request.

[LucasAdams]

What behaviour do you expect if you're being hit by 2+ people?

Should it be the same as the retaliate button or a different amount of troops?

[wiki-me]

I don't think there is a really good option for this. i think retaliating against the smallest attacking force so there won't be a risk of you spending all your population . i think it just sends troops to stop the attack and maybe a little more but i could be wrong.

If you need something more sophisticated then that then use the mouse.