#196933: iconBase="found/in/classpath.gif" does not work in JDK 6

http://netbeans.org/bugzilla/show_bug.cgi?id=196933
http://hg.netbeans.org/core-main/rev/5adfdac10656
http://hg.netbeans.org/core-main/rev/7e55a4244466

Impact: I was working on a demonstration module which used action icons from modules/ext/jlfgr-1_0.jar and referred to them from @ActionRegistration. While this ought to 
be legal, and works fine in Beta 2, now these actions will not compile, due to an attempt to verify that referenced icons really exist - which only works for icons in the 
source tree and not in the classpath, due to a bug in the version of javac in JDK 6.

Fix: use a weaker form of the icon existence check which always passes in JDK 6 whether the icon exists or not (due to another aspect of the same bug) but which does in 
fact perform the existence check when run using JDK 7 javac.

Note that a bug in the IDE's Java parser currently causes an error badge to appear although the Ant build succeeds: https://netbeans.org/bugzilla/show_bug.cgi?id=197104