Modify row-selected.png to match Gtk2/3 Treeview effects.

row-selected.png is modified to create the same visual effect for GTK2
treeview as is present in GTK3 (i.e. in Nemo).

This creates the "panel-board" look which provides item definition
without distracting white spaces.

OLD:
![](http://i.imgur.com/E6hcEyU.png)

NEW:
![](http://i.imgur.com/4BwEVEe.png)

Note: GTK3 already creates this effect for Nemo and other GTK3 applications, this pull is bringing GTK2 in to line.