Add support for basic project files

This add support for saving and loading of SQLiteBrowser project files.
As of now these files contain a reference to the used database file,
store some window and widget settings as well as the content of the SQL
tabs.

Note that while working this is a first draft only. Especially the
parsing code still might contain a bug or two and there is a lot more
information to be added to these files later (like filters, plot
settings, etc.).

Also note that the main intention behind these project files was to make
the life of users easier while not getting into the way of those who don't
need them. The program still remains a database browser and doesn't
become a visual database studio thing.