examples/squeezer_bin: Mark properties as writable

[bilelmoussaoui]

Fixes #1757

[sdroege]

error: this call to `as_ref` does nothing
  --> examples/squeezer_bin/squeezer_bin/imp.rs:27:37
   |
27 |         let widget = widget.map(|w| w.as_ref());
   |                                     ^^^^^^^^^^ help: try: `w`
   |