Base structure.
Called to run a file chooser dialog. |mode| represents the type of dialog to display. |title| to the title to be used for the dialog and may be NULL to show the default title ("Open" or "Save" depending on the mode). |default_file_path| is the path with optional directory and/or file name component that should be initially selected in the dialog. |accept_filters| are used to restrict the selectable file types and may any combination of (a) valid lower-cased MIME types (e.g. "text/*" or "image/*"), (b) individual file extensions (e.g. ".txt" or ".png"), or (c) combined description and file extension delimited using "|" and ";" (e.g. "Image Types|.png;.gif;.jpg"). To display a custom dialog return true (1) and execute |callback| either inline or at a later time. To display the default dialog return false (0).