idle_inhibit: Store wlr inhibitor instead of view

When an application inhibited idle, a view pointer was stored and a
destroy listener was registered to the wlr inhibitor. As the wlr
inhibitor lives longer than the view, this lead to a dangling view
pointer between view unmap and inhibitor destroy.

Store a pointer to the wlr inhibitor instead of to the view, and look up
the view when needed, which may at any point be NULL. This also allows
for an inhibitor to remain functional if a surface is re-mapped.

include/sway/desktop/idle_inhibit_v1.h

@@ -22,6 +22,7 @@ struct sway_idle_inhibit_manager_v1 {
 
 struct sway_idle_inhibitor_v1 {
 	struct sway_idle_inhibit_manager_v1 *manager;
+	struct wlr_idle_inhibitor_v1 *wlr_inhibitor;
 	struct sway_view *view;
 	enum sway_idle_inhibit_mode mode;