Fixup d207e85c: paint sublabel with the correct background color

Fixup d207e85c: paint sublabel with the correct background color

The above mentionned patch did paint the background of the sublabel to
avoid "bold-like" effect. However the correct backgound color is the
row's one, not the inset's one.

To fix this, extend MetricsInfo::backgroundColor to work when no inset
is specified. The code is also simplified as in master.

(cherry picked from commit 74540c9896892b8f956f52258a59613af83a5cf8)
(cherry picked from commit a71b96ac426438abb4fe835d0ad000c769693421)