Fixed all JFace errors encountered on my Fedora 24.

Previously Tree/Table was returning parent foreground color if
foreground == null. Overloaded getContextColor() so that foreground
color would be set to COLOR_LIST_FOREGROUND instead of
COLOR_WIDGET_FOREGROUND when foreground == null.

Change-Id: I0807a80a475ed942c3d72712a72e37d56724a45d
Signed-off-by: Ian Pun <ipun@redhat.com>