GUI: Improve project handling

- [ ] Merge top-level tabs "Welcome" and "Project"??

• In Welcome view, besides "recent projects" also show "current project"
• Besides buttons "New Project" and "Open Project", also provide "Save Project" and "close Project"

- [ ] Default save location: instead of ~/UntitledN/Untitled.pro, use ~/bornagain/projectN.pro.(reconsider when there are no more data files to be stored besides the project file)