Add association between wxOSX wxTextWidgetImpl and wxTextEntry.

Add association between wxOSX wxTextWidgetImpl and wxTextEntry.

This allows to always find the correct wxTextEntry to use in the
implementation of text-related widgets without using any casts. Notably, the
wrong up-cast of wxWindow to wxTextCtrl in wxNSTextFieldControl::controlAction()
which resulted in a crash when the window was actually a wxComboBox can now be
fixed.

Closes #12284.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65129 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775