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().
The constructor now has zero arguments. showDirectories and showFiles
are now set using setter methods.
FileLister::browse is now the sole way to start a scan for files and
directories, replacing the initial path in the constructor and the
setPath method. It allows merging results from multiple directories
as before.
This is all to make explicit how many times the costly task of browsing
a directory is actually carried out.
