Setup UI within feature UI

The feature is not enabled yet.
It will be enabled by default later when ui is ready.

build.rs

@@ -0,0 +1,9 @@
+use std::path::PathBuf;
+use std::env;
+
+fn main() {
+    println!("cargo:rerun-if-changed=gui/mainview.fl");
+    let g = fl2rust::Generator::default();
+    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
+    g.in_out("gui/mainview.fl", out_path.join("mainview.rs").to_str().unwrap()).expect("Failed to generate rust from fl file!");
+}