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/mod.rs

@@ -0,0 +1,23 @@
+/* Copyright (C) 2022 Purism SPC
+ * SPDX-License-Identifier: GPL-3.0+
+ */
+
+/*! Actors are parts of Squeekboard containing state independent from the main application state.
+
+Because main application state is meant to be immutable,
+it cannot be referenced directly by pieces of logic
+interacting with the environment.
+
+Such impure logic is split away (actor's logic)
+and combined with relevant pieces of state (actor state),
+thus preserving the purity (and sometimes simplicity) of the main state.
+
+Actors can communicate with the main state by sending it messages,
+and by receiving updates from it.
+*/
+
+// TODO: move crate::panel into crate::actors::panel.
+// Panel contains state and logic to protect the main state from getting flooded
+// with low-level wayland and gtk sizing events.
+
+pub mod popover;