Qt5 IBus input context does not implement hide_preedit_text()

IBus engines that calls "hide_preedit_text()" via D-Bus, which includes ibus-mozc, does not work correctly on Qt5 applications because Qt5 does not implement that function.

With regard to ibus-mozc, users cannot delete the last character in a preedit text. It is expected to be hidden by that function.

I have implemented hide_preedit_text and show_preedit_text function.
However, the current specification of Qt input context is not clear (the current implementation is a bit strange to me).

The "visible" attribute is not used and sending an empty string is the right way to hide preedit?

The original report is found at:
https://bugzilla.opensuse.org/show_bug.cgi?id=947013