client/gtk2: Use button-press-event only with IBUS_ENGINE_PREEDIT_COMMIT
client/gtk2: Use button-press-event only with IBUS_ENGINE_PREEDIT_COMMIT

BUG=https://github.com/ibus/ibus/issues/1980

ibus/ibus,fujiwarat/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus