Fix derive setter for default_icon_theme.

Added `strip_option` to prevent compile time error.

```bash
error[E0277]: the trait bound `Option<String>: From<&str>` is not satisfied
  --> examples/application/src/main.rs:24:29
   |
24 |         .default_icon_theme("Pop")
   |          ------------------ ^^^^^^^^^ the trait `From<&str>` is not implemented for `Option<String>`
   |          |
   |          required by a bound introduced by this call
```

src/app/settings.rs

@@ -26,7 +26,7 @@ pub struct Settings {
     pub(crate) default_font: Font,
 
     /// Name of the icon theme to search by default.
-    #[setters(into)]
+    #[setters(into, strip_option)]
     pub(crate) default_icon_theme: Option<String>,
 
     /// Default size of fonts.