Add methods to get the position of a window's client area, relative to the desktop (#430)

* Add get_inner_position for windows, prototypes for other platforms

* Fix linux builds

* Implement get_inner_position for osx

* Add get_inner_pos implementations for other platforms

* Fixed get_inner_position on macOS

* Corrected set_position on macOS

* Added CHANGELOG entry

src/platform/linux/mod.rs

@@ -162,6 +162,14 @@ impl Window {
         }
     }
 
+    #[inline]
+    pub fn get_inner_position(&self) -> Option<(i32, i32)> {
+        match self {
+            &Window::X(ref m) => m.get_inner_position(),
+            &Window::Wayland(ref m) => m.get_inner_position(),
+        }
+    }
+
     #[inline]
     pub fn set_position(&self, x: i32, y: i32) {
         match self {