Provide a checkbox component for embedding in a table.

Provide a checkbox component for embedding in a table.

This is needed by the Refspecs page. Unfortunately the code does not
work fine with Eclipse 3.3 (no image show) and the screen scraping approach
flickers horribly under OSX so we provide our own icon (stolen from the
Eclipse CVS archive at
pde-incubator/visualization/plugins/org.eclipse.draw2d/src/org/eclipse/draw2d/images)
plus some graying out for the disbled version, thereby solving the licensing issue.

Set the system property egit.swt.checkboxhack to switch behavior.
-Degit.swt.checkboxhack=hardwired will use the supplied icons without regard for
look and feel. -Degit.swt.checkboxhack=screenshot will grab the L&F from the
native SWT checkbox.

Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>