application: Use gio::ActionEntryBuilder for actions

Allows us to drop the action! macro and usage of the clone! macro
in the application file.

The first occurrence of clone! is now in window.rs, so the comment
explaining it has moved there.

src/window.rs

@@ -159,6 +159,14 @@ mod imp {
                 .chain_property::<LpImage>("best-fit")
                 .watch(
                     glib::Object::NONE,
+                    // clone! is a macro from glib-rs that allows
+                    // you to easily handle references in callbacks
+                    // without refcycles or leaks.
+                    //
+                    // When you don't want the callback to keep the
+                    // Object alive, pass as @weak. Otherwise, pass
+                    // as @strong. Most of the time you will want
+                    // to use @weak.
                     glib::clone!(@weak obj => move || {
                         let enabled = obj
                             .imp()