Before this patch following would cause a use-after-free:
- calling the Debug action when there was at least one view around
- closing that view
- calling the Debug action again
Also add a label for the layer popup tree.
- Store a pointer to the `struct view` in `struct ssd`
- Pass `struct ssd *` instead of `struct view *` to ssd functions
- Add `ssd_get_margin()` convenience function
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).
Support returning the following structs:
- view from LAB_NODE_DESC_VIEW or LAB_NODE_DESC_XDG_POPUP
- lab_layer_surface form LAB_NODE_DESC_LAYER_SURFACE
- lab_layer_popup from LAB_NODE_DESC_LAYER_POPUP
