Fix namespace isolation in GDCLASS macro

[touilleMan]

fix #918

Note the second commit in this PR modify the test to remove the use of using namespace godot;, from my point of view this has both pros and cons:

• pros: help detect namespace isolation leak like we saw here ^^
• pros: further show the difference between macros and regular functions (given macros are totally separated from namespace), this is important for D_METHOD which looks like a macro but is in fact a method (hence the GDCLASS(...) vs godot::D_METHOD(...))
• cons: the example is more verbose and less idiotic which is not great given it is the primary example one will refer to when learning Godot-CPP...

So what do you think ?
I guess the best of both worlds would be to have two separates folders: one for example and another for test (the latter typically being also run in the CI, which is something not currently done). However I'm personally too lazy for this change 😄

[akien-mga]

I think it's better if the example uses using namespace godot;, as it makes the code very similar to code in C++ modules, and being able to go easily from C++ module to GDExtension and back is a design goal for godot-cpp.

But indeed, we should probably consider separating the CI testing environment from the end user demo project. The current project was meant more for testing but ended up being used as reference demo.

[touilleMan]

@akien-mga I've removed the second commit then 👍

[akien-mga]

Thanks!