Backends: OSX: Fix keyboard support. Handle scroll cancel. Don't set mouse cursor shape unconditionally. (#4759, #4253, #1873)

Note the original FIXME: refered to GLFWs Cocoa implementation, which is largely what this commit provides.

examples/example_apple_metal/main.mm

@@ -119,7 +119,7 @@
         return event;
     }];
 
-    ImGui_ImplOSX_Init();
+    ImGui_ImplOSX_Init(self.view);
 
 #endif
 }