gschema: Introduce the 'app-picker-layout' key

gschema: Introduce the 'app-picker-layout' key

This is the key that will be used to store the pages and the icons in
each page. The idea is that we we store an a{sv} variant for each page.
This variant will contain <icon id> → array of properties, where we
can store arbitrary data for each icon. The expected output of this
key is:

[
  {
    'polari.desktop': <{ 'position': 0 }>,
    'epiphany.deskop': <{ 'position': 1 }>,
  },
  {
    'telegram.desktop': <{ 'position': 2 }>,
    'builder.desktop': <{ 'position': 0 }>,
    'gitg.desktop': <{ 'position': 1 }>,
  }
]

The toplevel array is sorted, and pages of the grid always show in the
order they are stored.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1284