Removed an assert in QxtTableWidget in order to fix QxtGui unit tests to pass. Added a note to docs that QTableWidget::itemEditingStarted() may carry a null pointer as a parameter.
Removed an assert in QxtTableWidget in order to fix QxtGui unit tests to pass. Added a note to docs that QTableWidget::itemEditingStarted() may carry a null pointer as a parameter.

mmitkevich/libqxt,hrobeers/qxtweb-qt5,hrobeers/qxtweb-qt5,mmitkevich/libqxt,hrobeers/qxtweb-qt5,mmitkevich/libqxt,hrobeers/qxtweb-qt5,mmitkevich/libqxt,hrobeers/qxtweb-qt5