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

include/foreign-toplevel/foreign.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_FOREIGN_TOPLEVEL_H
+#define LABWC_FOREIGN_TOPLEVEL_H
+
+struct view;
+struct foreign_toplevel;
+
+struct foreign_toplevel *foreign_toplevel_create(struct view *view);
+void foreign_toplevel_set_parent(struct foreign_toplevel *toplevel,
+	struct foreign_toplevel *parent);
+void foreign_toplevel_destroy(struct foreign_toplevel *toplevel);
+
+#endif /* LABWC_FOREIGN_TOPLEVEL_H */