view: fix some inconsistencies in view_ functions

... especially regarding whether a (view *) parameter may be NULL. It's
confusing when some functions accept NULL and others don't, and could
trip someone up.

I'm partly to blame for the inconsistency, since (if memory serves) I
added view_is_tiled() and view_is_floating(), which do accept NULL.

In detail:

- Make view_is_tiled() and view_is_floating() no longer accept NULL.

- Rename view_isfocusable -> view_is_focusable for consistency with
  other view_is_ functions.

- Eliminate view_inhibits_keybinds() as it only existed to safely accept
  NULL and check a single flag, which can be checked directly.

- Add assert(view) to remaining public view_ functions to catch
  accidentally passing NULL.

- Inline inhibit_keybinds() into view_toggle_keybinds(). It is closely
  related and not called from anywhere else; inlining it allows
  eliminating an extra assert() which is now impossible.

src/osd.c

@@ -479,7 +479,7 @@ osd_update(struct server *server)
 
 	/* Outline current window */
 	if (rc.window_switcher.outlines) {
-		if (view_isfocusable(server->osd_state.cycle_view)) {
+		if (view_is_focusable(server->osd_state.cycle_view)) {
 			osd_update_preview_outlines(server->osd_state.cycle_view);
 		}
 	}