Dispose the bold font on group disposal.

Change-Id: I5491c6f7cebb430fb21a94c2c1ed34f551a43531
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/181863
Tested-by: Platform Bot <platform-bot@eclipse.org>