footage: merge video and image streams and improve image sequence import process

Image streams were initially separated from video streams, but they're now
joined with a parameter defining if they're a still image, image sequence,
or regular video. The image sequence import process has also improved so
that if images from the same sequence are imported too, they'll either be
ignored or the user won't be asked again for those if they should be an
image sequence (fixes #1193). Also shifts decoder "probe" process to return
an item, useful if the decoder returns a non-footage item.

app/codec/ffmpeg/ffmpegdecoder.h

@@ -146,7 +146,7 @@ public:
   // Destructor
   virtual ~FFmpegDecoder() override;
 
-  virtual bool Probe(Footage *f, const QAtomicInt *cancelled) override;
+  virtual ItemPtr Probe(const QString& filename, const QAtomicInt* cancelled) const override;
 
   virtual bool Open() override;
   virtual FramePtr RetrieveVideo(const rational &timecode, const int& divider) override;