Cocoa: Start using instancetype

The first tiny step towards using more modern Objective-C.

src/cocoa_window.m

@@ -228,13 +228,13 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
     _GLFWwindow* window;
 }
 
-- (id)initWithGlfwWindow:(_GLFWwindow *)initWindow;
+- (instancetype)initWithGlfwWindow:(_GLFWwindow *)initWindow;
 
 @end
 
 @implementation GLFWWindowDelegate
 
-- (id)initWithGlfwWindow:(_GLFWwindow *)initWindow
+- (instancetype)initWithGlfwWindow:(_GLFWwindow *)initWindow
 {
     self = [super init];
     if (self != nil)
@@ -381,13 +381,13 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
     NSMutableAttributedString* markedText;
 }
 
-- (id)initWithGlfwWindow:(_GLFWwindow *)initWindow;
+- (instancetype)initWithGlfwWindow:(_GLFWwindow *)initWindow;
 
 @end
 
 @implementation GLFWContentView
 
-- (id)initWithGlfwWindow:(_GLFWwindow *)initWindow
+- (instancetype)initWithGlfwWindow:(_GLFWwindow *)initWindow
 {
     self = [super init];
     if (self != nil)