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().

src/filedialog.h

@@ -29,7 +29,6 @@ public:
 			GMenu2X *gmenu2x, Touchscreen &ts, const std::string &text,
 			const std::string &filter="*", const std::string &file="",
 			const std::string &title = "File Dialog");
-	virtual ~FileDialog();
 	bool exec();
 };