Added GetClientRect (#702)

Added GetClientRect function to get the size of the Window excluding the borders.

CHANGES.md

@@ -59,6 +59,7 @@ Features
 * [#667](https://github.com/java-native-access/jna/pull/667): Added NtSetSecurityObject and NtQuerySecurityObject to `com.sun.jna.platform.win32.NtDll` - [@amarcionek](https://github.com/amarcionek).
 * [#680](https://github.com/java-native-access/jna/pull/680): Added `SetCurrentProcessExplicitAppUserModelID` and `GetCurrentProcessExplicitAppUserModelID` to `com.sun.jna.platform.win32.Shell32` for setting the [System.AppUserModel.ID](https://msdn.microsoft.com/en-us/library/windows/desktop/dd391569.aspx) of the host process - [@rednoah](https://github.com/rednoah).
 * [#693](https://github.com/java-native-access/jna/pull/693): Bind DDEML (Dynamic Data Exchange Management Library), add a thread implementation that runs a windows message loop - [@matthiasblaesing](https://github.com/matthiasblaesing).
+* [#702](https://github.com/java-native-access/jna/pull/702): Added `GetClientRect` to `com/sun/jna/platform/win32/User32` - [@Jonatino](https://github.com/Jonatino).
 
 Bug Fixes
 ---------

contrib/platform/src/com/sun/jna/platform/win32/User32.java

@@ -188,11 +188,30 @@ public interface User32 extends StdCallLibrary, WinUser, WinNT {
      *            Long pointer to a RECT structure that receives the screen
      *            coordinates of the upper-left and lower-right corners of the
      *            window.
-     * @return Nonzero indicates success. Zero indicates failure. To get
-     *         extended error information, call GetLastError.
+     * @return If the function succeeds, the return value is nonzero. If the
+     *         function fails, the return value is zero.
      */
     boolean GetWindowRect(HWND hWnd, RECT rect);
 
+    /**
+     * This function retrieves the coordinates of a window's client area. 
+     * The client coordinates specify the upper-left and lower-right corners of the 
+     * client area. Because client coordinates are relative to the upper-left 
+     * corner of a window's client area, the coordinates of the upper-left corner 
+     * are (0,0).
+     *
+     * @param hWnd
+     *            Handle to the window.
+     * @param rect
+     *            Long pointer to a RECT structure that structure that receives 
+     *            the client coordinates. The left and top members are zero. The 
+     *            right and bottom members contain the width and height of the 
+     *            window.
+     * @return If the function succeeds, the return value is nonzero. If the
+     *         function fails, the return value is zero.
+     */
+    boolean GetClientRect(HWND hWnd, RECT rect);
+
     /**
      * This function copies the text of the specified window's title bar - if it
      * has one - into a buffer. If the specified window is a control, the text