API change: ParseRunnerPlugin: remove guiString(), introduce...

API change: ParseRunnerPlugin: remove guiString(), introduce fileFormatDescription() and fileExtensions()

The pure virtual method guiString() wasn't used in Marble and was only there for historical reasons.

The new pure virtual methods fileFormatDescription() and fileExtensions() represent metadata which can be used for automatic filter creation in a file open dialog.
That way, the file open dialog could offer all available file formats, depending on which plugins are installed.