Desktop: Make embedded resources optional (#3094)

* Make embedding resources optional

* Move remaining cef rc to internal module

* Move embedded resources to separate crate

* Review fixup

* Fix

* Fix read

* Add read error

desktop/embedded-resources/build.rs

@@ -0,0 +1,17 @@
+const RESOURCES: &str = "../../frontend/dist";
+
+// Check if the directory `RESOURCES` exists and sets the embedded_resources cfg accordingly
+// Absolute path of `RESOURCES` available via the `EMBEDDED_RESOURCES` environment variable
+fn main() {
+	let crate_dir = std::path::PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap());
+
+	println!("cargo:rerun-if-changed={RESOURCES}");
+	if let Ok(resources) = crate_dir.join(RESOURCES).canonicalize()
+		&& resources.exists()
+	{
+		println!("cargo:rustc-cfg=embedded_resources");
+		println!("cargo:rustc-env=EMBEDDED_RESOURCES={}", resources.to_string_lossy());
+	} else {
+		println!("cargo:warning=Resource directory does not exist. Resources will not be embedded. Did you forget to build the frontend?");
+	}
+}