[Tails-dev] Make it possible to manually start IBus on all l…

We switched from Scim to IBus, see tickets #5624 and #6206. Among the
changes was to always start IBus by default, regardless of login locale.
But problems with keyboard shortcuts in for example France locale (see
#6478) required a quick fix to not always start IBus by default.

However, after this quick fix it is no longer possible for a user to start
IBus *at all* in locales other than CJK locales. My branch
"winterfairy:bugfix/allow-ibus-start" (based on devel) fixes this issue by
allowing the user to start IBus manually, but still not starting IBus by
default for non-CJK locales. Therefore resolving this issue while avoiding
to hit #6478 again.

The design documentation is also updated.

AFAIK, there is no ticket tracking this regression. The original issue was
a subitem in #5624.

Tested with (by patching Tails 0.22.1):
- Chinese (IBus started by default, everything works)
- Japanese (IBus started by default, everything works)
- Korean (IBus started by default, everything works)
- Swedish (IBus can be started manually)
- France (IBus can be started, and shortcuts works if it is not)

Please review and merge.