feat(app): add show_close for toggling close button in header bar

src/app/core.rs

@@ -36,6 +36,7 @@ pub struct Window {
     pub show_context: bool,
     pub show_headerbar: bool,
     pub show_window_menu: bool,
+    pub show_close: bool,
     pub show_maximize: bool,
     pub show_minimize: bool,
     height: u32,
@@ -128,6 +129,7 @@ impl Default for Core {
                 sharp_corners: false,
                 show_context: false,
                 show_headerbar: true,
+                show_close: true,
                 show_maximize: true,
                 show_minimize: true,
                 show_window_menu: false,