ui/crumbs/persistenttooltip: fix colours on darker themes

ui/crumbs/persistenttooltip: fix colours on darker themes

Darker gtk+ themes, such as the one used in Ubuntu Unity, revealed that the
PersistentTooltip styling wasn't setting the label colour correctly.

Set the label foreground colour to the tooltip_fg_colour value as read from
gtk-color-scheme property of the system settings.

(From Poky rev: 0934cfcea5986dbdc50e7159ee907c70b0b3e587)

(Bitbake rev: ab15ef585e51e4c85a4a55aa6b35fbf3b53f3805)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>