Fix handling of wxTextEntry hints in wxGTK.

Fix handling of wxTextEntry hints in wxGTK.

Recent changes (r71308) optimizing away setting the text of the text entry if
it didn't really change completely broke the generic text hints implementation
as e.g. clearing the hint didn't change anything because the entry was already
logically empty anyhow.

Fix this by simply comparing the new value with the real value of the entry
returned by DoGetValue() and not GetValue() that takes hints into account.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72458 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775