Gather related logic from `reload_config_and_theme()` in `server.c` and
`ssd_reload()` in `ssd.c` into a new function, `view_reload_ssd()`.
Also drop the `view->mapped` check since we want to update any view that
has SSD nodes created, mapped or not.
Add xdg_surface_from_view() + xwayland_surface_from_view() accessors
that assert() the view is of the expected type before returning.
Fix a real bug in xdg.c parent_of() that dereferenced
`view->xdg_surface->toplevel` without first checking `view->type`.
The goal of the new accessors is to catch similar bugs in future.
IMHO it encourages better design (by making dependencies more obvious)
to have source file/header file pairs like view.c/view.h, rather than a
monolithic header like labwc.h with everything in it.
I don't think we need to break up all of labwc.h at once, but maybe we
can start pulling it apart bit by bit as it's convenient.
Also:
- Move "struct border" to ssd.h so that view.h can use it without pulling
in all of labwc.h.
- Add a missing required #include within scaled_font_buffer.h (forward
declaration of "struct font" is not enough).
