Adapt IBus configuration for Jessie (Closes: #8270)

Here, we merge the two places where we configure keyboard layout and input
methods: both are now configured in the same place in Jessie's GNOME.
Don't add the US keyboard layout twice in English (Closes: #7997).
Drop tails-save-im-environment.

It's not been used since we stopped automatically starting the web browser.
Adjust design doc to match current reality.
Migrate panel launchers to the favorite apps list (Closes: #7992).
Drop pre-GNOME Shell menu tweaks.