Use a plain FileLister instead of a pointer in BrowseDialog

Configuration of the FileLister is now left to the subclasses of
BrowseDialog, as the construction of a FileLister and its configuration
(to show or hide directories and files) have been separated.

This allows deleting the destructors of BrowseDialog's subclasses,
which existed solely to delete a FileLister object constructed by
each subclass.

The allocation of a BrowseDialog now also implies allocating enough
space for its FileLister, so remove the check for fl being nullptr in
BrowseDialog::exec().