Add keyboard navigation for the back button

Thanks to Joanmarie Diggs for the suggestion.

https://bugzilla.gnome.org/show_bug.cgi?id=724980
Disallow selection in GlEventView

The details view is show when a row is activated, so do not allow
selection.
Switch category when a new row is selected

Change the current category based on the row that is selected, not when
a row is activated.

https://bugzilla.gnome.org/show_bug.cgi?id=724980
Set browse selection mode on GlCategoryList
Ignore tab focus direction inside category list

Make a tab forward or back to another child within the category list
instead jump out of the list and focus the next child of the window.
This improves keyboard navigation, by making the listbox behave more
like a treeview.

https://bugzilla.gnome.org/show_bug.cgi?id=724980