refactored window storage;

new helper window events (Destroyed, Created);
clippy + fmt;

core/src/window/settings/macos.rs

@@ -0,0 +1,12 @@
+//! Platform specific settings for macOS.
+
+/// The platform specific window settings of an application.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct PlatformSpecific {
+    /// Hides the window title.
+    pub title_hidden: bool,
+    /// Makes the titlebar transparent and allows the content to appear behind it.
+    pub titlebar_transparent: bool,
+    /// Makes the window content appear behind the titlebar.
+    pub fullsize_content_view: bool,
+}