keyboard: Don't press and release keyval if it's a modifier key

keyboard: Don't press and release keyval if it's a modifier key

Looks like we currently do the press-and-release dance for keys that specify
both a keyval and the "modifier" action. So when pressing CTRL in the
extended OSK, not only is the CTRL modifier set together with the next key
pressed, but the actual CTRL keyval quickly gets pressed and released,
similar to how we'd do it for the TAB key.

This seems unintentional, as the press-and-release behavior should probably
be specific to keys like TAB, and not apply to modifier keys, so limit that
behavior to only keys without an action specified.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3286>
(cherry picked from commit cae8b745cab97056d7fee9aa30053817f44f53ad)