Fixes to comparison operators for wxDVC classes.

Fixes to comparison operators for wxDVC classes.

Comparison operators for wxDataViewItem and wxDataViewIconText were not inline
and not exported, resulting in linking errors for any code using them in
shared wx build. Fix this by making them inline.

Also correct wxDataViewIconText operator==() implementation to compare icons
as well and to return true when comparing the object with itself.

Finally add operator!=() matching existing operator==() as a class having one
of these operators is supposed to have the other one as well and it costs
nothing to define it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62401 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775