Don't override DoGetSize, just let the default return the real size of

Don't override DoGetSize, just let the default return the real size of
the widget like normal.  Since the former implementation used
GetBestSize, which uses a DC, which on Mac calls window->GetSize, this
resulted in an infinite recursion.

Also use SetBestFittingSize in the ctor and move CacheBestSize to
DoGetBestSize where it belongs.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42409 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775