[Added] Strife conversation reply cursor graphic replacement

[Nash]

https://github.com/coelckers/gzdoom/pull/1134

As requested by this user.

If "graphics/DialogReplyCursor.png" is present, it will be used for the reply cursor. Otherwise, the default ConFont cursor is used. This ensures that there is no change to existing mods.

[Graf Zahl]

Do you have something to test it with? The feature is all nice and well but without the ability to review it there's very little chance to see it added.

[Nash]

Demo file. Fastest way to test this is to just load it up with Strife and talk to the first guard outside the first door.

[Major Cooke]

Merged into QZDoom for testing.