Return smaller images for wxART_MENU/BUTTON under OS X.

Return smaller images for wxART_MENU/BUTTON under OS X.

Requesting images with client id of wxART_MENU/BUTTON used to return the large
32*32 icons because GetNativeSizeHint() wasn't implemented for these client
ids.

Moreover, under Mac some icons (notably message box ones) are created from the
corresponding icon bundle and the code in wxArtProvider::GetBitmap() didn't
resize them correctly in this case, fix this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62299 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775