Layout differences due to context

@dorota.czaplejewicz asked to pin down my thoughts in a bug:

As discussed in the meeting there's several things to cater for:

• a language has a primary layout
• these have multiple levels (see e.g. 123 key in the mockups https://gitlab.gnome.org/Teams/Design/os-mockups/raw/master/mobile-shell/mobile-keyboard.png)
• depending on the context (email, gnumeric, ...) the levels have slightly different keys (so e.g. when entering an email you want the @ in the primary level).