Prefer AccountsService's InputSources property instead of custom KeyboardLayout/XkbOptions pair.

[Marukesu]

Currently (inspecting the system bus) we keep keyboard state in two AccountsService interfaces:

• io.elementary.pantheon.AccountsService (ActiveKeyboardLayout, and KeyboardLayouts).
• io.elementary.SettingsDaemon.AccountsService (ActiveKeyboardLayout, KeyboardLayouts, and XkbOptions).

At the same time, the standard org.freedesktop.Accounts.User interface has a InputSource property that can replace both KeyboardLayouts and XkbOptions properties from our custom interfaces.

[Marukesu]

So, i did a little search about it, and seems to be a ubuntu patch (proposed upstream here).

it stores a array of dictionaries, the keys are ibus, xkb, and xkb-options. currently i have this value in my system (probably set by GSD):

~$ busctl --system get-property org.freedesktop.Accounts /org/freedesktop/Accounts/User1000 org.freedesktop.Accounts.User InputSources
aa{ss} 2 1 "xkb" "br" 1 "ibus" "xkb:br::por"

The major issue is that this is a Ubuntu patch, so maybe we want to mirror it in another interface for downstream usage (at least, i known that fedora doesn't include it).