Add wxCompositeWindow<> and use it in wxDatePickerCtrlGeneric.

Add wxCompositeWindow<> and use it in wxDatePickerCtrlGeneric.

wxCompositeWindow<> is a convenient base class for composite windows, i.e.
windows consisting of several other wxWindows. Currently it just automatically
forwards various attributes setters calls to all of the composite window parts
but it could become more useful in the future.

Similarly, for now it is only used in wxDatePickerCtrlGeneric but it could
(and should) be used for other composite controls later and we probably should
even make this class public to allow its use in the client code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66534 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775