Tweaked the key mappings so that it is now easier to change key mappings and update the corresponding UI elements as well

src/app/key_binding_tests.rs

@@ -0,0 +1,18 @@
+#[cfg(test)]
+mod test {
+  use crate::app::key_binding::{build_keymapping_string, DEFAULT_KEYBINDINGS};
+  use pretty_assertions::assert_str_eq;
+
+  #[test]
+  fn test_build_keymapping_string() {
+    let test_keys_array = [
+      (DEFAULT_KEYBINDINGS.add, "add"),
+      (DEFAULT_KEYBINDINGS.delete, "delete"),
+    ];
+
+    assert_str_eq!(
+      build_keymapping_string(&test_keys_array),
+      "<a> add | <del> delete"
+    );
+  }
+}