Add ellipsization support to wxDataViewCtrl.

Add ellipsization support to wxDataViewCtrl.

Implemented ellipsization in the generic, GTK and both OS X Carbon and Cocoa
versions but it currently doesn't work well in GTK as it changes the item
alignment unconditionally, this will need to be fixed later.

The behaviour for the columns is currently inconsistent between ports too:
under MSW they (natively) use wxELLIPSIZE_END, under GTK -- wxELLIPSIZE_NONE
and under OS X the same ellipsization mode as the column contents, i.e.
wxELLIPSIZE_MIDDLE by default.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62433 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775