New style & property routines.

New routine to get all styles for all CSS files referenced by a page.
New routine to get CSS property value.
Check CSS styles for hiding labels

Check CSS styles used to hide labels or move labels off screen.
Don't report WCAG_2.0-ARIA12 error for tags that have role="heading" if
it also has an aria-level attribute greater than 6.
Include last heading in labels and legends to distinguish similar labels
- WCAG_2.0-H44 and WCAG_2.0-H71.
Coding change  for substring check

Change code used to determine if strings are a substring of each other.
Remove unused routine.
Report runtime error

Add routine to report runtime error if the analysis thread of the tool
fails.