Fix wxTextEntry::SelectAll() in presence of hints in wxGTK.

Fix wxTextEntry::SelectAll() in presence of hints in wxGTK.

Translation of wx (-1, -1) selection to (0, GetValue().length()) in
wxTextCtrl::SetSelection() was unnecessary as it ended up calling the base
class wxTextEntry::SetSelection() version which didn't need it. Moreover, this
translation was actually harmful when the text control happened to show a hint
string as its official value was empty in this case and so SetSelection(0, 0)
was called which didn't do anything and broke clearing/changing the controls
text when it was showing a hint.

Simply don't translate the indices when using a single line control to fix
this.

See #12475.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65550 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775