Make screenshot selection work again

This is a workaround for some yet-to-be-diagnosed GTK+
gesture fallout. The stack we use for each screenshot has
a window. Setting the event mask explicitly ensures that we
get button events, which will then bubble up the listbox
which maintains the selection.

https://bugzilla.gnome.org/show_bug.cgi?id=735712
Focus the search entry on keypress

And don't steal the focus from the entry when the results appear

https://bugzilla.gnome.org/show_bug.cgi?id=735274