Define keybinding via css to expand/collapse with arrows. Tested to work
on GTK 3.20. Adjusted selector for previous version.

Leo:
Small adjustment:
< Gtk3.20 :  gtk-key-bindings
>=Gtk3.20 : -gtk-key-bindings
Tested:
- Works on Gtk3.8, 3.10, ... 3.20, 3.22(beta)
- CSS has no effect on <=Gtk3.6 (but 3.6 is 4 years old)

Change-Id: Ifa0a0b226e0003d55c44e2e1a4093e09e275fe1e
Signed-off-by: Alexander Kurtakov <akurtako@redhat.com>
Signed-off-by: Lev Ufimtsev <lufimtse@redhat.com>