Project Browser - sort and display modification dates

The Project Browser currently displays cells in an essentially arbitrary order.  If the user has many models and/or stores available due to testing in various simulator versions, it can be difficult to find the combination you’re looking for.

The two related changes made here:

1. When the browser loads/reloads, the data is sorted prior to creation of the cells.  The Store modified date is the primary sort, with the Model being secondary.  The most common use case is likely to open the most recently modified store and model pair.  That pair is now at the top of the Project Browser.

2. Each cell now displays the relative modification dates (13 minutes ago, 2 months ago...) in the Model and Store labels.  This further assists the user in determining which item to open.

In both cases above, both the main Store file and any associated .sqlite-wal (write-ahead log) files are used to establish the Store modification date.