This is a common practice in C projects, which simply enforces that
each header must compile cleanly without implicit dependencies on
other headers (see also the previous commit).
This matches the behavior of the surrounding borders and will usually
cause resizing instead of triggering the button itself. This effect
is only really visible when using a border width settings of > 1.
Fixes#379
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