[JBIDE-13939] fix usage tests for Kepler M6
* Ensure GlobalUsageSettingsTest is restoring the usage enablement in
  the system properties after test execution.
* Dont run UsageReport within the display thread in IStartup since UsageReport
  already switches to the display thread where required
* Added tycho configuration to have this run outside the UIHarness, UIThread
* Removed catch in UsageReportDispatcher since it extends SafeRunnable
JBIDE-14009 Change palette content when HTML page is changed

Method update() is added to PaletteContents to be used by a
document listener to switch palette.