Fixed an issue with viewer not getting focus in gtk back end
Fixed an issue with viewer not getting focus in gtk back end

- viewer should normally get the keyboard focus if the cursor
  enters the window

naojsoft/ginga,ejeschke/ginga,naojsoft/ginga,pllim/ginga,ejeschke/ginga,pllim/ginga,ejeschke/ginga,pllim/ginga,naojsoft/ginga