Navigation: Reduce include dependencies with forward declares

[akien-mga]

• Helps address [TRACKER] Compile time issues caused by excessive includes #111218

I checked manually all navigation_* and nav_* headers to see what includes could be removed or replaced by forward declares, to reduce incremental recompilation time.

[akien-mga]

That's the only non-navigation change in the PR. I initially set out to minimize includes of scene headers in servers headers, which is how I fell into this navigation rabbit hole.

[akien-mga]

Seems like MSVC doesn't like the use of forward declared types in RID_Owner, unlike GCC and Clang:

Error: .\core/templates/rid_owner.h(431): error C2027: use of undefined type 'NavObstacle2D'
D:\a\godot\godot\modules\navigation_2d\2d/godot_navigation_server_2d.h(42): note: see declaration of 'NavObstacle2D'
.\core/templates/rid_owner.h(431): note: the template instantiation context (the oldest one first) is
D:\a\godot\godot\modules\navigation_2d\2d/godot_navigation_server_2d.h(81): note: see reference to class template instantiation 'RID_Owner<NavObstacle2D,false>' being compiled
.\core/templates/rid_owner.h(518): note: see reference to class template instantiation 'RID_Alloc<T,false>' being compiled
        with
        [
            T=NavObstacle2D
        ]
.\core/templates/rid_owner.h(424): note: while compiling class template member function 'RID_Alloc<T,false>::~RID_Alloc(void)'
        with
        [
            T=NavObstacle2D
        ]
Error: .\core/templates/rid_owner.h(94): error C2079: 'RID_Alloc<T,false>::Chunk::data' uses undefined class 'NavObstacle2D'
        with
        [
            T=NavObstacle2D
        ]
.\core/templates/rid_owner.h(94): note: the template instantiation context (the oldest one first) is
.\core/templates/rid_owner.h(93): note: while compiling class 'RID_Alloc<T,false>::Chunk'
        with
        [
            T=NavObstacle2D
        ]
Error: .\core/templates/rid_owner.h(439): error C2325: 'T': unexpected type to the right of '.~': expected '<error type>'
        with
        [
            T=NavObstacle2D
        ]

[akien-mga]

I reverted those forward declares since MSVC doesn't like them.
godot_navigation_server_*.h is only included in register_types.cpp anyway so forward declares here weren't particularly beneficial.

[StarryWorm]

Total includes project-wide go from 187973 to 187461 with this PR¹, so -512 or -0.27%. That's actually a kind of huge improvement for such a small change.

LGTM!

_{¹ Data obtained by cherry-picking the commit to a new branch off of master as the PR branch is a bit behind. Numbers only reflect total includes by *.cpp files.}

[akien-mga]

This may have been missing in the master branch too, I found it by compiling this PR with target=template_release disable_navigation_2d=yes disable_navigation_3d=yes.