client/gtk2: Do not delete IBUS_CAP_SURROUNDING_TEXT
client/gtk2: Do not delete IBUS_CAP_SURROUNDING_TEXT

retrieve-surrounding signal could be failed with the first typing
on firefox. It could be a bug in firefox but now IBusIMContext does not
delete IBUS_CAP_SURROUNDING_TEXT in the capabilities as a workaround
when retrieve-surrounding signal is failed.
Also added retrieve-surrounding signal after some committing text.

BUG=https://github.com/ibus/ibus/issues/2054

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