gtk: Improve menu appearance.

This disables icons in menus (we weren't showing them for
most menus but were still using the ugly GTK defaults in the
omnibox) and cuts down on the huge amount of empty space to
the left of menu items' text.

(Note that the excessive spacing is still present in the
wrench menu, though.)

Change-Id: I421cc29ff2108ac5f478e380409d0ec50735c28f

TEST=build and tried it

Review URL:
1 file changed