Fix setting initial size of wxPropertyGridManager.

wxPG manager is now created with proper size (based on its best size) even if default size is requested. (r78149 is a prerequisite for this patch.)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78150 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775