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/manager.rs

@@ -0,0 +1,18 @@
+/*! Procedures relating to the management of the switching of layouts */
+
+pub mod c {
+    use std::os::raw::{c_char, c_void};
+
+    /// EekboardContextService*
+    #[repr(transparent)]
+    #[derive(Clone, Copy)]
+    pub struct Manager(*const c_void);
+    
+    #[no_mangle]
+    extern "C" {
+        pub fn eekboard_context_service_set_overlay(
+            manager: Manager,
+            name: *const c_char,
+        );
+    }
+}