Make more realistic estimation of the best size of wxPropertyGridManager.

Current best size (especially width) of wxPropertyGrid Manager is too small. wxPG manager should be wide enough to hold wxPG with two columns and scroll bar.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78151 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775