Header inclusion minimization

[TheJJ]

In order to reduce compile times and prevent cyclic includes in the first place, all our files should be revisited:

• In all header files, only non-pointer or non-reference data types are included from another header
• All pointer or reference data types are forward declared in each header (class Bla;)
• In the corresponding c++ file, those incomplete types are imported by #include "header"

Thus, complete types are only included in files where they are needed.
Template stuff and inheritance parents etc must be included in header files directly of course.

[MaanooAk]

This could help:

find . -iname *.h -exec cppclean "{}" \;  | grep "#include"

To install cppclean

pip install cppclean

[VelorumS]

@MaanooAk, some false positives if the traits are in separate headers.

[MaanooAk]

@ChipmunkV I know, If you somehow pass all the files in one command, should be ok but it may crash... Not perfect in any way. I just used it recently in a much smaller project and thought could help if someone tries to actually to fix the headers...

[mic-e]

We should definitely include cppclean as part of the code compliance checker. Does it produce any false positives?

[zuntrax]

First file checked was a false positive (not needing config.h in the assetmanager). Also crashes every few files. Might be nice for cleanups, but sadly not suitable for codecompliance.

[mic-e]

Damnit... have i mentioned how the move to modules would solve everything™?

[zuntrax]

We had some discussion about this back in #263. The idea here was not forward declaring manually, but having declaration headers.

[zuntrax]

Also we should think about explicit instantiation declaration. http://en.cppreference.com/w/cpp/language/class_template

[heinezen]

The iwyu run in #1525 probably resolved most of this.