Add option to open with last used project

From the feedback-form: "Please allow to auto-open previous project in Qt Designer. Just waste of time to open it every time from the menu. Just like Eclipse does."

[He means "Qt Creator".]

I personally would never use that option as one always ends up loading the last big project only to switch to something else, but the issue comes up regularly, so the one-time effort for just implementing the option seems to be less effort than continuously fighting over it. Default should be "off", and it should be remembering a session, not a project.

dt might have a strong opinion on the issue.