Javadoc body's background

Part 2:
Javadoc hover should set colors of the Hover (not just HTML content),
and update them when the user changes "Javadoc" properties.

Tests:
- Open child Eclipse. Click on Editor (actually click on editor, not
just hover over, otherwise text 'press F2 to focus' is not set.
- Hover mouse over method that has javadoc.
- Strip at the bottom has same color as javadoc.

- Change Javadoc colors in User preference. Observe that
the whole javadoc hover changes color.

KNOWN LIMITATION:
- Currently gtk-dark theme overrides the color of the hover with it's
own colors. But it looks OK. To be investigated in another patch.

Patchset 2:
- Refactored duplicate code into setHoverColors() method.
  (Strings to be converted to constants in followup patch).
- Fixed a small bug in propertyChange(), added missing brackets.

Change-Id: I1f9ffbd785ccf818fe0e53d410f811548cd2c450
Signed-off-by: Leo Ufimtsev <lufimtse@redhat.com>