popover: Install emoji layout

In order to do that, an additional piece of state (layout switcher) was exposed to the event handlers, a separation between squeekboard-only and system layouts was introduced, along with a Translation structure to prevent mixing up strings.

src/lib.rs

@@ -25,6 +25,7 @@ mod layout;
 mod locale;
 mod locale_config;
 mod logging;
+mod manager;
 mod outputs;
 mod popover;
 mod resources;