Bug#926452: gnome-shell: On-screen keyboard lacks several French layouts

[intrigeri]

Control: severity -1 important
Control: retitle -1 gnome-shell: On-screen keyboard lacks several French layouts

I've verified that this is a regression compared to Stretch,
where the visual keyboard supports fr_FR.UTF-8 just fine:
the displayed layout matches a fr_FR physical keyboard's
and one can input accentuated chars that are part of this layout,
such as "à" or "é" ⇒ bumping severity.