new: iterator interface and improved documentation for JKQTPDatastore
reorganization of library (better separation of common code in jkqtpcommon and other code e.g. in jkqtplotter or jkqtmathtext)
new: user-interaction tool that measures distances and angles when mouse is dragged, while mouse button is pressed)
update: reworked error indicator base classes and styling settings
update: improved boxplot styling with a separate full-featured pen for the whiskers
