Save/restore main window layout

[PierreRaybaut]

Currently, DataLab is only saving and restoring the size, position and maximized state of the main window. This is serialized in the configuration file (in user's home folder), not in the workspace HDF5 file.

It would be interesting to add the following new feature to DataLab:

• [x] Save/restore toolbars and dock widgets positions to/from configuration file
• [ ] Save/restore window settings with the workspace HDF5 file
• [ ] Save/restore toolbars and dock widgets positions with the workspace HDF5 file