state: Become the source of layout choice

A redesign of popover was needed: it can no longer query the application state directly due to current state being its own actor, so instead the popover gets a dedicated copy of the relevant state.

I'm not entirely happy with the extra complexity of having an extra actor just for 1 string, but at least the duplication between C and Rust and mutual calls have been reduced.

src/actors/popover.rs

@@ -0,0 +1,40 @@
+/* Copyright (C) 2022 Purism SPC
+ * SPDX-License-Identifier: GPL-3.0+
+ */
+
+/*! The popover is opened directly by the GTK surface,
+without bouncing click events off the main state.
+Then it must accurately show which layout has been selected.
+It can get the system layout directly from gsettings on open,
+but it cannot get the user-selected overlay, because it's stored in state.
+
+To solve this, overlay will be cached in the popover actor,
+and updated by main state every time it changes.
+*/
+
+pub mod c {
+    use super::*;
+    use crate::util::c::Wrapped;
+    /// The mutable instance of state
+    pub type Actor = Wrapped<State>;
+}
+
+#[derive(Clone)]
+pub struct State {
+    pub overlay: Option<String>,
+}
+
+impl State {
+    pub fn new() -> Self {
+        Self { overlay: None }
+    }
+}
+
+pub fn set_overlay(
+    actor: &c::Actor,
+    overlay: Option<String>,
+) {
+    let actor = actor.clone_ref();
+    let mut actor = actor.borrow_mut();
+    actor.overlay = overlay;
+}