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

Reworked for new DBus API and event-driven plugin interface to QML.

Essentially, we now get only one DBus signal with id when the layout
changes.
All the other info applet is capable to lookup locally via it's own data
- no need for additionally DBus calls any more.

This is possible because new API provides exhaustive info about all
layouts via updated getLayoutsList() call - now it returns a vector of
all layouts with all names for each. This info is stored on QML side in
Actions objects - which at the same time serve to fill context menu.