* Implemented BestSize cache

* Implemented BestSize cache

* Added calls to InvalidateBestSize where things affecting BestSize
  are modified.  There are probably several other places where this
  still needs to be done...

* Added wxWindowBase::GetBestFittingSize that will merge the BestSize
  into the MinSize, (if any) and return the result.

* SetBestFittingSize will now only set the MinSize to the value that
  was passed to it, without merging in the BestSize

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@28010 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775