IBus input of Japanese and Korean characters is broken in Tor Browser

Reported by several users. Still needs to be tested on Debian:

Since Tails 3.6rc1 users cannot input Japanese text in Tor Browser. It works in other programs in Tails, such as the Text Editor, but it does not work on TBB nor the Unsafe Browser.

To reproduce:

Start Tails in Japanese with Japanese keyboard
Write things in Text editor and Tor browser.

Corresponding bug reports: 43c31f44a97dda68c59989049f0d9c59, bfee9b4c138c78ae1c809794588f861f

Feature Branch: bugfix/15437-IBus-in-Tor-Browser, torbrowser-launcher.git:bugfix/15437-IBus-in-Tor-Browser

Related issues

Blocks #13245 (closed)

Original created by @emmapeel on 15437 (Redmine)

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