Merged PR #816 force to use "using namespace godot;" in namespace where GDCLASS macro is used

[ComradeNixco]

One might not want to "polute" the namespace by having to use using namespace godot; in a header file. Fix would be to use godot::* for each class used in the macro that comes from the godot namespace

[ComradeNixco]

The PR where the "breaking" change occurs: #816

[Calinou]

cc @bruvzg

[Faless]

Fixed via #823