foreign-toplevel: simplify and fully separate ext-foreign/wlr-foreign

foreign-toplevel: simplify and fully separate ext-foreign/wlr-foreign

Currently, the dependencies between foreign-toplevel[-internal],
ext-foreign, and wlr-foreign are cyclical and a bit complex.

I suggest we reorganize it into a simpler hierarchy:

  foreign-toplevel/
    -> foreign.c/h
      -> (depends on) ext-foreign.c/h
      -> (depends on) wlr-foreign.c/h

The refactored code is smaller and (IMO) easier to follow.

In detail:

- Add include/foreign-toplevel folder mirroring src/foreign-toplevel
- Split foreign-toplevel-internal.h to ext-foreign.h and wlr-foreign.h
- Eliminate ext-/wlr-foreign.c -> foreign.c reverse dependencies
  (including internal signals and foreign_request* functions)
- Make struct foreign_toplevel private to foreign.c

Lightly tested with qmpanel (which uses wlr-foreign-toplevel).

v2: reorder foreign-toplevel internal API funcs