Make keyboard match mockups

Feature: https://source.puri.sm/Librem5/Apps_Issues/issues/65

There wasn't a bug but what I gather from @heather.ellsworth and @tobias.bernard it should match the images in virtboard#14. Since this bug is about reveal and hide lets separate these.

Design mockups:

https://gitlab.gnome.org/Teams/Design/os-mockups/blob/master/mobile-shell/mobile-keyboard.png