Add support for entering emoji by name

Add support for entering emoji by name

This commit adds some basic support for entering emoji by name
to GtkIMContextSimple. To begin an emoji sequence, use Ctrl-Shift-e
instead of Ctrl-Shift-u that is used for hex input. Otherwise, the
behavior is the same: you can can let go of the modifier keys and
end the sequence with space or enter, or hold on to the modifier
keys and end the sequence by releasing them.

Only a limited, fixed set of names is supported at this time, see
the GtkIMContextSimple docs for a full list.