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/x11/window.rs

@@ -919,6 +919,11 @@ impl Window2 {
         self.get_geometry().map(|geo| geo.get_position())
     }
 
+    #[inline]
+    pub fn get_inner_position(&self) -> Option<(i32, i32)> {
+        self.get_geometry().map(|geo| geo.get_inner_position())
+    }
+
     pub fn set_position(&self, mut x: i32, mut y: i32) {
         if let Some(ref wm_name) = self.wm_name {
             // There are a few WMs that set client area position rather than window position, so