Layer surface and gi-docgen related cleanups

Things that came about when working on phosh#764 (closed)

The layer-surface code has several oddities that make quick coding cumbersome:

• uses layer to identify the layer enum or to identify a PhocLayerSurface or to identify a wlr_layer_surface
• still uses roots_ prefix a lot
• use no prefix for struct wlr_layer_surface which makes it easy to confuse with PhocLayerSurface

So work towards calling PhocLayerSurface * layer_surface and struct wlr_layer_surface * wlr_layer_suface and the enum zwlr_layer_shell_v1_layer layer.

These patches make things more consistent in that regard for places I had to touch.