[macOS] Move the window if there is no title bar (#382)

* macOS: Move the window if there is no title bar

On macOS by default windows can only be moved by clicking and
dragging on the titlebar, if we spawn a window without one we
need to set the `movableByWindowBackground` property.

Partial fix for #368

* macOS: Make moveByWindowBackground optional

Implements setting the property via WindowBuilderExt:

    WindowBuilder::new()
        .with_decorations(false)
        .with_movable_by_window_background(true)

* Update CHANGELOG

src/platform/macos/window.rs

@@ -254,6 +254,7 @@ impl Drop for WindowDelegate {
 #[derive(Clone, Default)]
 pub struct PlatformSpecificWindowBuilderAttributes {
     pub activation_policy: ActivationPolicy,
+    pub movable_by_window_background: bool,
 }
 
 pub struct Window2 {
@@ -312,7 +313,7 @@ impl Window2 {
             None      => { return Err(OsError(format!("Couldn't create NSApplication"))); },
         };
 
-        let window = match Window2::create_window(win_attribs)
+        let window = match Window2::create_window(win_attribs, pl_attribs)
         {
             Some(window) => window,
             None         => { return Err(OsError(format!("Couldn't create NSWindow"))); },
@@ -381,7 +382,10 @@ impl Window2 {
         }
     }
 
-    fn create_window(attrs: &WindowAttributes) -> Option<IdRef> {
+    fn create_window(
+        attrs: &WindowAttributes,
+        pl_attrs: &PlatformSpecificWindowBuilderAttributes)
+        -> Option<IdRef> {
         unsafe {
             let screen = match attrs.fullscreen {
                 Some(ref monitor_id) => {
@@ -449,6 +453,10 @@ impl Window2 {
                     window.setTitlebarAppearsTransparent_(YES);
                 }
 
+                if pl_attrs.movable_by_window_background {
+                    window.setMovableByWindowBackground_(YES);
+                }
+
                 if screen.is_some() {
                     window.setLevel_(appkit::NSMainMenuWindowLevel as i64 + 1);
                 }