Update changelog, comments, made empty/no-text clipboard return NULL as with other implementation. Minor style tweaks. (#2546)

Fixed IMGUI_DISABLE_WIN32_FUNCTIONS not disabling IME code.

examples/imgui_impl_osx.mm

@@ -12,6 +12,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2019-05-18: Misc: Removed clipboard handlers as they are now supported by core imgui.cpp.
 //  2019-05-11: Inputs: Don't filter character values before calling AddInputCharacter() apart from 0xF700..0xFFFF range.
 //  2018-11-30: Misc: Setting up io.BackendPlatformName so it can be displayed in the About Window.
 //  2018-07-07: Initial version.
@@ -55,6 +56,9 @@ bool ImGui_ImplOSX_Init()
     io.KeyMap[ImGuiKey_Y]           = 'Y';
     io.KeyMap[ImGuiKey_Z]           = 'Z';
 
+    // We don't set the io.SetClipboardTextFn/io.GetClipboardTextFn handlers,
+    // because imgui.cpp has a default for them that works with OSX.
+
     return true;
 }