The dirty indicator bullet inherited the tab text foreground color, which
only varies between selected and unselected tabs in dark themes via CSS.
In light themes the bullet looked identical on active and inactive tabs.

Apply a reduced alpha (140) when drawing the bullet on unselected tabs so
the active/inactive distinction works in any theme without CSS changes.

Fixes https://github.com/eclipse-platform/eclipse.platform.swt/issues/3278