Gui improvments
This MR fixes and improves several issues concerning GUI improvements
#2 (closed) The application is now always launched in light theme
#3 (closed) The QLineEdit
+QDoubleValidator
have been replaced by QDoubleSpinBox
which handled automatically the locale settings
#4 (closed) The log files are now written in the application directory (e.g. ~/.softais on Unix)
#5 (closed) The NXStress files are not checkable anymore in the data tree. For that purpose, the reference dialog was refactored.
#6 (closed) The header contextual menu was moved to the table contextual menu with enabling/disabling of some of the items depending on the number of columns selected