Unbreak X11 copy/paste, support wp_single_pixel_buffer and some tidying around the edges

* cursor: Use G_UNLIKELY for debug flag

* cursor: Fix indentation

* cursor: Use explicit cast

* layer-shell-effects: Fix typo

  Gbp-Dch: Ignore

* desktop: Reindent phoc_desktop_constructed

  Make use of the full line length and give every protocol that also
  registers signals a small block to ease readbility.
  
  This also gets rid of tabs in that function which made indenting
  harder than necessary.

* desktop: Enable support for wp_single_pixel_buffer protocol

* desktop: Fix formatting of handle_xwayland_*

* Set xwayland seat when XWayland comes up

  Since XWayland can run on demand setting it only on server startup
  is not enough.
  
  This unbreaks copy/paste between X11 and Wayland applications as
  selection handling doesn't work without an assigned seat.
  
  It also prevents emacs from hanging on close when trying to safe the
  clipboard.

Actual changes are tiny but as ever so often this needs a bit of cleanup in phoc's codebase (that improved considerably though over the past iterations and whitespace churn gets smaller)

Closes: #326 (closed)