Adding uninstallColors and uninstallColorsAndFont LookAndFeel

[Liam Miller-Cushon]

Hello,

While investigating JDK-8277817 [1], it was discovered that many of the
existing callers of LookAndFeel.{installColors,installColorsAndFont} fail
to correctly uninstall the installed properties.

[1] https://bugs.openjdk.java.net/browse/JDK-8277817

As part of the fix for that bug, I would like to add
corresponding uninstallColors and uninstallColorsAndFont methods to
LookAndFeel, to make it easier for the affected callers to
correctly uninstall properties installed by the existing methods.

The draft CSR is available at
https://bugs.openjdk.java.net/browse/JDK-8278555

The review showing the fix for JDK-8277817 using the proposed methods is
here: https://git.openjdk.java.net/jdk/pull/6603

I'd appreciate any feedback.

Liam