...to increase xwayland and xdg-shell encapsulation and to avoid passing a
function pointer as an argument in `xwayland_move_sub_views_to_front()`
which is inconsistent with labwc design patterns.
Rename view-impl.c to view-impl-common.c
Move function declarations that are common to view-implementations from
view.h into view-impl-common.h
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).
