wxPGProperty::AddChild() can now be used to add normal child properties (previously...

wxPGProperty::AddChild() can now be used to add normal child properties (previously it was only used to add private children of derived property classes such as wxFontProperty and wxFlagsProperty). However, to allow backwards compatibility, SetParentalStyle(wxPG_PROP_MISC_PARENT) needs to be called before doing so. Also done: merged property initialization code from PrepareToAddItem() and PrepareSubProperties() to InitAfterAdded().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56309 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775