Help IDEs resolve gen/include immediately

[van800]

Create the godot-cpp/gen/include directory at configure time so IDEs can see generated headers immediately after generate_bindings target produces them.

The problematic use-case, which I have fixed for Rider, CLion and VS:

1. clone a fresh copy of godot-cpp-template
2. open example_class.cpp
   observe red code
3. call generate_bindings target
   still observe red code, while expected result would be to see all resolved.

Notes:

• maybe this should be done somewhere in the godot-cpp, not sure.
• It would be even nicer to call the generate_bindings target in the design time, but that would be arguable bigger change.

[van800]

@enetheru, what do you think?

[enetheru]

is there changes missing? how does creating that directory effect anything?

[van800]

I suppose, if folder does not exist, IDE can't watch it for further changes. I haven't dug too deep, but I checked the behavior is consistent with Clion, Rider and VS.

[enetheru]

I suppose, if folder does not exist, IDE can't watch it for further changes. I haven't dug too deep, but I checked the behavior is consistent with Clion, Rider and VS.

well if it helps i'm all for it.