src: style: fix build with newer gtk-rs

`gtk::Settings::property()` no longer returns a `Result` but instead
panics if the property doesn't exist. In order to work around this
change without affecting the surrounding code too much, this patch
modifies the `prop` function so we can avoid panics and wrap the
property value into a `Result` as was previously the case.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/620>

src/style.rs

@@ -99,7 +99,12 @@ fn get_theme_name(settings: &gtk::Settings) -> GtkTheme {
         }).ok();
 
     #[cfg(feature = "glib_v0_14")]
-    let prop = |s: &gtk::Settings, name| s.property(name);
+    let prop = |s: &gtk::Settings, name| {
+        if s.has_property(name, None) {
+            return Ok(s.property_value(name));
+        }
+        return Err("Key not found in settings");
+    };
     #[cfg(not(feature = "glib_v0_14"))]
     let prop = |s: &gtk::Settings, name| s.get_property(name);