Bug 1966427 - Don't use native splitter drawing on GTK. r=desktop-theme-reviewers,dao

GTK4 paned widgets (see gtk4-demo) are only a thin line.

For now this does the windows thing as that's low-risk, which looks
basically the same as we have now for most themes.

We can make it look nicer in the future if needed.

Differential Revision: https://phabricator.services.mozilla.com/D249394

servo/components/style/values/specified/box.rs

@@ -1547,9 +1547,6 @@ pub enum Appearance {
     /// The down button of a spin control.
     #[parse(condition = "ParserContext::chrome_rules_enabled")]
     SpinnerDownbutton,
-    /// A splitter.  Can be horizontal or vertical.
-    #[parse(condition = "ParserContext::chrome_rules_enabled")]
-    Splitter,
     /// A status bar in a main application window.
     #[parse(condition = "ParserContext::chrome_rules_enabled")]
     Statusbar,