Dive media: on import read metadata only once

On import of dive media, the timestamp is read from the
metadata to check if the image belongs to the selected dives.
The pictures are then listed in a dialog.

Currently, the metadata is read twice if images are outside
of a dive: once in picture_check_valid() and if it turns
out that the picture is not valid again in picture_get_time()
to display the proper timestamp.

Even though metadata-extraction is reasonably fast, this is
a bit of an embarrassment.

Instead, read the timestamps only once in the constructor of
the dialog and from then on only used these timestamps. Keep
the timestamps in a QVector. Rename the picture_check_valid()
function to picture_check_valid_time() and pass a timestamp
instead of a filename.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/simplewidgets.h

@@ -118,6 +118,7 @@ slots:
 
 private:
 	QStringList fileNames;
+	QVector<timestamp_t> timestamps;
 	Ui::ShiftImageTimesDialog ui;
 	time_t m_amount;
 	time_t dcImageEpoch;