additional info pane

Part 2:
AdditionalInfoProposal (i.e, code-completion javadoc helper window)
should use javadoc colours defined in user preferences. (Fixes styling).

This affects win32/cocoa/gtk in the sense that the popup will use
Javadoc colours in user-preference. However, default win32/cocoa
configurations won't notice the change because COLOUR_INFO_BACKGROUND is
used as default anyway. This does fix the way the popup looks on Gtk
Dark theme thou. (See attached screenshot)

Test:
- Open child eclipse, turn on dark theme. Restart eclipse.
- Type "Obj", press ctrl+space until "Object" appears.
- Inspectect "helper javadoc" side window.

With patch: Popup receives colours from user preference "Javadoc"
Before patch: Uses default HTMLPrinter colours (which aren't nice in
dark theme).

Change-Id: I5b785169c9ae5186d11a729598905e03d585210d
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=507073
Signed-off-by: Leo Ufimtsev <lufimtse@redhat.com>