input: remove the concept of "significant" modifiers

For the purpose of matching key bindings, "significant" modifiers are
no more.

We're really only interested in filtering out "locked"
modifiers. We're already doing this, so there's no need to *also*
match against a set of "significant" modifiers.

Furthermore, we *never* want to consider locked keys (e.g. when
emitting escapes to the client application), thus we can filter those
out already when retrieving the set of active modifiers.

The exception is the kitty keyboard protocol, which has support for
CapsLock and NumLock. Since we're already re-retrieving the "consumed"
modifiers (using the GTK style, rather than normal "XKB" style, to
better match the kitty terminal), we might as well re-retrieve the
effective modifiers as well.

input.h

@@ -33,6 +33,6 @@ void input_repeat(struct seat *seat, uint32_t key);
 void get_current_modifiers(const struct seat *seat,
                            xkb_mod_mask_t *effective,
                            xkb_mod_mask_t *consumed,
-                           uint32_t key);
+                           uint32_t key, bool filter_locked);
 
 enum cursor_shape xcursor_for_csd_border(struct terminal *term, int x, int y);