Split platform specific "C"-bindings into the separate files

[bruvzg]

Currently, accesskit.h includes some Obj-C headers when it is used on macOS, therefore it is usable only from the Obj-C/Obj-C++ source, but not from pure C/C++.

Splitting platform specific adapter bindings to the separate file would allow using platform independent part of the code from C/C++.

Or alternatively, since no Obj-C specific stuff is really used in the binding, it might be worth getting rid of the Obj-C includes, by replacing NSObject * with void * (already done with NSView * for subclassing adapter) and CGPoint with two floats.

[DataTriny]

Thanks @bruvzg for the report.

I thought this could work.

We will go the second path then, as there is not an easy way to generate multiple header files with cbindgen.