StringBuilder instead of StringBuffer in
HTMLPrinter

Used upcoming JDT cleanup to replace this usage in PDE, StringBuilder is
not synchronized and hence should be preferred.

Change-Id: I7d141dc73d61ef1135c912c2025a822cde5cf021
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.text/+/181565
Tested-by: Platform Bot <platform-bot@eclipse.org>