Enhanced behaviour of username widget. It now has a list of known
users (based on subfolders of the data folder whose names are not
datestamps), automatically adds a new user when the name is set and
shows the remembered previous user in the GUI upon startup (as well as
adding it to the list of known users).
Modified one line so that the vbox in the main window doesn't
immediately build itself after adding the username widget, since it is
not ready at this point and will (re-)build itself later anyway. This
modification will shave valuable microseconds off the time it takes to
initialise QTLab.
Merge branch 'master' of https://github.com/zeusttu/neqstlab
Extra sanity checking for datadir existence in populating user combo box