Don't change wxTextEntry value if it didn't really change.

This is not only unnecessary but also can result in a kind of infinite loop
when wxTextEntry::SetValue() is called from wxEVT_IDLE handler as really
modifying the text results in another wxEVT_IDLE being generated and another
call to SetValue() and so on while not doing it in the first place completely
works around the problem and is also more efficient.

Closes #13936.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71308 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775