Examine the desktop module's use of sun.misc.SoftCache

The desktop module has a single remaining dependency on jdk.unsupported, for use of sun.misc.SoftCache. SoftCache
is not a critical API, as defined by JEP 260, nor is it used widely inside or outside the JDK. In fact, the desktop module
is the only user of it.

jdeps -v java.desktop/* | grep unsupported
   sun.awt.SunToolkit -> sun.misc.SoftCache JDK internal API (jdk.unsupported)
   sun.awt.image.MultiResolutionToolkitImage -> sun.misc.SoftCache JDK internal API (jdk.unsupported)
   sun.awt.image.MultiResolutionToolkitImage$ObserverCache -> sun.misc.SoftCache JDK internal API (jdk.unsupported)

If the desktop module needs the functionality provided by SoftCache, then it should be moved to an appropriate internal
package in the desktop module.