BUG: Fix editor focus issues in SplitEditorAreaPane
BUG: Fix editor focus issues in SplitEditorAreaPane

 - Prev/Next tab shortcuts would not actually set focus on the
   new editor, instead if the editor pane receives focus again
   the previously focused widget would get activated
 - activate_editor() would not actually set focus on the
   activated editor, resulting in previously focused editor
   regaining focus when the editor pane is activated.

geggo/pyface,brett-patterson/pyface,pankajp/pyface,geggo/pyface