PDI-10420, PDI-10419 - add help button, modify help dialog

 + Added help button to XUL based step/job entry dialogs
 + Add Help to context menu for steps/job entries in View tab in Spoon
 * Show error message dialog if the documentation for step is not found
 * Show web help after user click Help button/ctxt menu item
Merge pull request #302 from askdzmitry/PDI-10420

PDI-10420, PDI-10419 - add help button, modify help dialog