Fix current field highlighting in generic wxTimePickerCtrl under wxGTK.

In wxGTK, calling HighlightCurrentField() from EVT_SET_FOCUS handler was not
enough as the highlighting was overwritten by the default behaviour of
selecting the entire text control contents on focus gain.

Fix this by calling SetFocus() from HighlightCurrentField() before updating
the selection. This ensures that the correct field is always highlighted.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69291 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775