Create a regression test for JDK-4231298
Issue:
When a JComboBox first drops down, the renderer is called for
every element in the list, even those elements that will not
actually be drawn on the screen. 

Fix:
Introduced new API to JComboBox: get/setPrototypeDisplayValue(). This API allows the UI to calculate the display size using this prototype value rather than iterated over a large list of items. See RFE 4289100 which also addresses this problem.

Testing:
1. Tested in Mach5, 10 times per platform(macos,linux and windows) and got all pass.

2. Tested in JDK 1.3.0 and JDK 1.4.0 and the results are given below.
Java 1.3.0 -> Test Failed.
$ jdk1.3/bin/java JComboBoxPrototypeTest
getListCellRendererComponent index = -1, isSelected = false, cellHasFocus = false
Value of the Counter is 1
....
getListCellRendererComponent index = -1, isSelected = false, cellHasFocus = false
Value of the Counter is 101
Test Failed
java.lang.RuntimeException: Custom Renderer got called 101 times, expected is maximum 6 times
        at JComboBoxPrototypeTestOld.main(JComboBoxPrototypeTest.java:107)

Java 1.4.0(added a call to setPrototypeDisplayValue()) -> Test Passed.
$ j2sdk1.4.0/bin/java JComboBoxPrototypeTest
getListCellRendererComponent index = -1, isSelected = false, cellHasFocus = false
Value of the Counter is 1
getListCellRendererComponent index = -1, isSelected = false, cellHasFocus = false
Value of the Counter is 2
getListCellRendererComponent index = -1, isSelected = false, cellHasFocus = false
Value of the Counter is 3
Test Passed