system: Align submenu labels with parent

system: Align submenu labels with parent

All section titles use an icon, while all corresponding submenu items
are plain-text. As a result, labels in submenus aren't aligned with
the labels in the parent, which makes them harder to read and look
unbalanced. Address this by adding additional whitespace to submenu
items to account for the additional elements in the title.

https://bugzilla.gnome.org/show_bug.cgi?id=706191