Core/Scripts: Re-add the modify currency command, accidentally removed in fca4a5448ac476b942c330108a76032ae242711e

Closes #11507

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Core/Players: Fixed Death Knight starting First Aid skill max value
Core/Guilds: Use the right opcode in Guild::SendCommandResult.

Documented the change a little.
Core/Dependencies: Clean up recastnavigation folder

Remove old files from recastnavigation folder and update the structure to the same of https://github.com/memononen/recastnavigation/ to allow easier updates.
Merge branch 'master' of github.com:TrinityCore/TrinityCore into 4.3.4

Conflicts:
	dep/PackageList.txt
Auth/Battle.net: Corrected some battle.net auth response enum values
Merge.