fix: keyboard layout applet: no flag if Display Name is set

If user set custom name for the layout, country flag is not displayed.
Instead, Display Name is shown in the applet.

This reveals shortcomings in current DBus API design.
We need more data to pass over DBus to fix this.
Then, multiple improvements are possible:
- fix aforementioned bug
- add flags to context menu
- display correct translated Layout Name in the context menu
- simpler, cleaner DBus API and applet implementation
- etc.