Support SurroundingText ibus capability

IBus has a special event to delete surrounding characters.
Previously we didn't support that event which forced the
IPA keyboard to send us a backspace instead. However I'm not sure
that all keyboards would work, so it seems safer and cleaner to
support surrounding text.

Change-Id: I51b8aa40d39ec3111b68d505cf0b7b10c7f7d0f3