st-im-text: Replace key-* handler with captured-event

When using an input method like IBus, the IM is expected to process
key events before anything else. Currently this doesn't always work
as expected, as the event filtering is done in the default handlers
of the key-press and key-release events, e.g. only after other
handlers have been run.
To allow the IM to filter events earlier, move the code to a
captured-event handler instead.

https://bugzilla.gnome.org/show_bug.cgi?id=658325