Added support to custom button labels on message box.

Changed the message when trying to save an invalidated model. (issue
More improvements on model validation warning. Added the button
"Validate".

Added support for custom icons on message box
Minor adjustment on model export dialog size.
Minor improvement on model overview widget.

Minor adjustments on window title buttons for model export and database
import forms.
Merge branch 'custom-msgbox-btn-lables' into develop