Expose window::Mode in iced

Although the Fullscreen API in the Web platform has some limitations, it
is still useful to be able to support fullscreen on the native side.

src/window/mode.rs

@@ -0,0 +1,9 @@
+/// The mode of a window-based application.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Mode {
+    /// The application appears in its own window.
+    Windowed,
+
+    /// The application takes the whole screen of its current monitor.
+    Fullscreen,
+}

src/window/settings.rs

@@ -0,0 +1,22 @@
+/// The window settings of an application.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Settings {
+    /// The size of the window.
+    pub size: (u32, u32),
+
+    /// Whether the window should be resizable or not.
+    pub resizable: bool,
+
+    /// Whether the window should have a border, a title bar, etc. or not.
+    pub decorations: bool,
+}
+
+impl Default for Settings {
+    fn default() -> Settings {
+        Settings {
+            size: (1024, 768),
+            resizable: true,
+            decorations: true,
+        }
+    }
+}