Allow setting locked field of choose-elem-button as an argument to .add()

[jagoly]

LuaGuiElement has a handy boolean option that prevents the user from modifying the value of a choose-elem-button. However, for some reason this can only be set after the element is created by doing element.locked = true. It would be much more convenient if this field could be set by adding locked = true to the argument table passed to parent.add().

[Rseding91]

Ok. Added for the next release.