Add wxDataViewIconText::IsSameAs() and make comparison operators members.

Add wxDataViewIconText::IsSameAs() and make comparison operators members.

Add IsSameAs() to make it simpler to call from the derived class operator==()
implementation.

Also make comparison operators themselves members instead of global functions
to avoid considering them as matches for all operator==() uses in the program,
there is really no need for this as we do _not_ want to allow implicitly
converting something to wxDataViewIconText when comparing.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68851 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775