This was broken in a86cf1ec (3.32) when the input options dialog was converted
into a GtkPopover. Previously two GSettings objects were used, with one set to
'delay-apply' mode. The input source option didn't apply the changes.

The 'delay-apply' mode is from the original commit d3852fc8, however code was
simplified in b3199dd6 to only set one setting. This meant that the delay is no
longer necessary.

Fixes #440