commands: Add per-view shortcuts_inhibitor command

Add a separate per-view shortcuts_inhibitor command that can be used
with criteria to override the per-seat defaults. This allows to e.g.
disable shortcuts inhibiting globally but enable it for specific,
known-good virtualization and remote desktop software or, alternatively,
to blacklist that one slightly broken piece of software that just
doesn't seem to get it right but insists on trying.

Add a flag to sway_view and handling logic in the input manager that
respects that flag if configured but falls back to per-seat config
otherwise. Add the actual command but with just enable and disable
subcommands since there's no value in duplicating the per-seat
activate/deactivate/toggle logic here. Split the inhibitor retrieval
helper in two so we can use the backend half in the command to retrieve
inhibitors for a specific surface and not just the currently focused
one. Extend the manual page with documentation of the command and
references to its per-seat sibling and usefulness with criteria.

Signed-off-by: Michael Weiser <michael.weiser@gmx.de>

sway/commands.c

@@ -127,6 +127,7 @@ static struct cmd_handler command_handlers[] = {
 	{ "rename", cmd_rename },
 	{ "resize", cmd_resize },
 	{ "scratchpad", cmd_scratchpad },
+	{ "shortcuts_inhibitor", cmd_shortcuts_inhibitor },
 	{ "split", cmd_split },
 	{ "splith", cmd_splith },
 	{ "splitt", cmd_splitt },