Add shortcut to append active skeleton to selection

Create a keyboard shortcut to append the active skeleton to the last
used selection widget. SelectionTable now tracks the last focused
selection widget. Currently this shortcut is bound to 'Y'.

Thanks to Anton Miroschnikow for the original feature request.
Selection widget: fix window not taking focus

Because it does not call addLogic, the selection widget window was not
taking focus. It displayed as focused, but did not remove focus from
other windows and did not trigger focus listeners.