WIP: config: Type safe API prototype

If we represent settings with types, we can provide an api to
get/set/monitor settings that guarantees we can only access settings
that exist with the right types, without seperate getter/setter/callback
functions like GTK would use.

cosmic-config/examples/app.rs

@@ -0,0 +1,27 @@
+use cosmic_config::setting::{App, Setting, AppConfig};
+
+struct ExampleApp;
+
+impl App for ExampleApp {
+    const ID: &'static str = "com.Example.App";
+    const VERSION: u64 = 1;
+}
+
+struct DoFoo;
+
+impl Setting<ExampleApp> for DoFoo {
+    const NAME: &'static str = "do-foo";
+    type Type = bool;
+}
+
+struct WhatBar;
+
+impl Setting<ExampleApp> for WhatBar {
+    const NAME: &'static str = "what-bar";
+    type Type = String;
+}
+
+fn main() {
+    let config = AppConfig::<ExampleApp>::new().unwrap();
+    config.set::<DoFoo>(true).unwrap();
+}