Tab name lengths over 31 chars corrupt Excel export

[wd_perf]

Export functionality in getWBook() should limit sheet names to 31 characters so that when the spreadsheet is opened in Excel, it does not cause an error. Excel requires tab names < 32 characters long.

I modified getWBook to call a getUniqueSheetName function that keeps track of used names and returns:

verylongnamethatcrashesexcelverylongnamethatcrashesexcel
anotherverylongnamethatcrashesexcelverylongnamethatcrashesexcel
athirdverylongnamethatcrashesexcelverylongnamethatcrashesexcel
-->
verylongnamethatcrashesexcelver
verylongnamethatcrashesexcelve2
verylongnamethatcrashesexcelve3

[paramvir]

Tabs UI doesn't allow tab names more than 31 characters.

This issue doesn't arise unless worksheet names are updated in the javascript workbook directly.