view/xwayland: avoid focusing views that don't want focus

view/xwayland: avoid focusing views that don't want focus

XWayland views can self-declare that they don't want keyboard focus via
the ICCCM WM_HINTS property. Most of the logic is already in place to
avoid giving focus to such views (e.g. taskbars).

Add a couple of missing pieces to make this work:

- Hook up view_isfocusable() to look at WM_HINTS for XWayland views
- Adjust desktop_focus_topmost_mapped_view() to skip unfocusable views