New SortRole needed for QFileSystemModel (and QSortFilterProxyModel)

QFileSystemModel::data() method only allows to return data from its 4 columns by using either the Qt::EditRole or the Qt::DisplayRole. It calls several other private methods depending on the column queried. Column 0 is file name, 1 is size, 2 is type, 3 is modified date. These methods appropriately return the corresponding value converted into a human-friendly QString.

On the other hand, by default QSortFilterProxyModel uses the Qt::DisplayRole to query the needed data from the model.

When using a QSortFilterProxyModel over a QFileSystemModel, the Qt::DisplayRole is not the correct role to be used, since comparison over a human-friendly QString is prone to be totally incorrect (and in fact it is, most of the times). QFileSystemModel::data() needs to support some "SortRole" to return raw (original typed) data for each column, so the proper sorting can be applied at QSortFilterProxyModel::lessThan().

Example:
Current behavior: when using a QSortFilterProxyModel over a QFileSystemModel, sorting by the 4th column will sort the date strings, but earlier dates won't necessarily come before later dates.
Desired behavior: earlier dates come always before later dates, independently of what is the string representation of dates chosen by the locale.

Similarly, sizes are transformed into SI standard magnitudes, thus not allowing a proper ordering.

Proposed solution is that QFileSystemModel::data() returns QString variants when the requested role is Display role (current behaviour), but returns raw (base type) values when role is SortRole (not implemented).

This SortRole would be set through the QSortFilterProxyModel::setSortRole() method.