keyboard: Propagate ratio changes to the emoji selection panel

Update the emoji panel so it can handle ratio changes dynamically,
and propagate the ratio from the Keyboard itself, so that the
emoji panel has a size that fits the OSK panel it was launched
from.

This is more important now with widely varying ratios, like
extended keyboards.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2278>