Allow the wielded item to be specified in commands

[marius851000]

The implementation for https://github.com/Uberi/Minetest-WorldEdit/issues/249

It use either w or wielded for using the user wielded object. It properly check for all possible case of error I can think of, and is not available in the GUI.

(I think that replacing the w or wielded text with the currently wielded item would avoid the complexity of refreshing the UI whenever the player change item, but it also mean understanding how to set the GUI searchbox.)

It’s the first time I code on a Minetest mod.

I modified the function signature in a way that should allow mods that does not provide the playername keep working, and mods that does provide the playername with an older worldedit version also works.