Support over-the-top display when entering multibyte characters using ibus/mozc.

[AiraYumi]

The Linux version of the official viewer has just been restarted, so the priority for this support is quite low.

Although it is supported by the Linux version of Firestorm viewer, I think it would be more appropriate to support it from the Linux version of the official viewer.

Before:
https://www.youtube.com/watch?v=fZJQDIXHC3I

After:
https://www.youtube.com/watch?v=L5-ojjOjsAw

[Nicky-D]

@AiraYumi the issue here is that the placement of the IME window is wrong, but the input itself works okay?
I've never used IME myself, this is why I want to make sure to understand the problem

[AiraYumi]

@Nicky-D There is no problem as long as you recognize that the input form is in the wrong position. The input functionality itself is not affected.

(very low priority) Ideally, the text would be displayed directly inside the input form, like Kate did.

example
https://www.youtube.com/watch?v=wWmvhW2wTjQ

[AiraYumi]

I have resolved the fcitx issue in #1427 but not the ibus issue.