Fixes a resource leak (GDI Handles) in createDisabledImage() in
org.eclipse.ui.forms.widgets.ImageHyperlink. Replaced redundant code in
setEnabled method. Removed useless if(image==null) and added a missing
redraw() in setImage() so that setImage() works similar to
setText(). Added a JUnit Test for the leak.

Change-Id: Iabb5c5bc345de5e707f89cdc0575f4ba604b8712
Signed-off-by: Ralf Petter <ralf.petter@gmail.com>
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>