Duplicated commit strings in some firefox sites

[Riey]

Example

https://discord.com/app

Summary

오[랜] + ㅁ = 오랜ㅁ랜ㅁ| not 오랜[ㅁ]

Detail

Due to #535, gtk frontend call update_preedit(FALSE) to end preedit state before commit

It's because chromium doesn't update preedit string when preedit string is same as before, but this can't handle [ㅋ] + ㅋ = ㅋ[ㅋ] case

So kime now always end preedit state before commit but this produced another bug.

In above site, it commits reset when preedit was ended (it's not always occurred but I'm not sure exact condition).
When this occurred in middle of commit process, it'll double commit string. Furthermore, preedit state also be terminated.

Note that this only affects develop branch, since #536 has not been released.