Generate clipboard events for wxComboBox in wxGTK too.

These events were only generated for wxTextCtrl but should be sent for
non-readonly wxComboBox too, so refactor the code to allow its reuse from
wxComboBox.

Also add EVT_TEXT_PASTE handlers for both controls to the widgets sample for
testing.

Closes #14520.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72252 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775