- for Qt 5.15: added missing #include statements that became necessary since Qt 5.15
- for Qt 5.15: fixed use of several deprecated functions
- for Qt 5.15/6: removed usage of QMatrix
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)