undo: implement undo of setting a picture time by drag&drop

Even though the functionality is seemingly trivial, this is a bit
invasive, as the code has to be split into two distinct parts:
1) Post undo command
2) React to changes to the divelist

Don't compile that code on mobile.

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

commands/command_pictures.cpp

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "command_pictures.h"
+#include "core/subsurface-qt/divelistnotifier.h"
+
+namespace Command {
+
+static picture *dive_get_picture(const dive *d, const QString &fn)
+{
+	int idx = get_picture_idx(&d->pictures, qPrintable(fn));
+	return idx < 0 ? nullptr : &d->pictures.pictures[idx];
+}
+
+SetPictureOffset::SetPictureOffset(dive *dIn, const QString &filenameIn, offset_t offsetIn) :
+	d(dIn), filename(filenameIn), offset(offsetIn)
+{
+	if (!dive_get_picture(d, filename))
+		d = nullptr;
+	setText(Command::Base::tr("Change media time"));
+}
+
+void SetPictureOffset::redo()
+{
+	picture *pic = dive_get_picture(d, filename);
+	if (!pic) {
+		fprintf(stderr, "SetPictureOffset::redo(): picture disappeared!");
+		return;
+	}
+	std::swap(pic->offset, offset);
+	offset_t newOffset = pic->offset;
+
+	// Instead of trying to be smart, let's simply resort the picture table.
+	// If someone complains about speed, do our usual "smart" thing.
+	sort_picture_table(&d->pictures);
+	emit diveListNotifier.pictureOffsetChanged(d, filename, newOffset);
+	invalidate_dive_cache(d);
+}
+
+// Undo and redo do the same thing
+void SetPictureOffset::undo()
+{
+	redo();
+}
+
+bool SetPictureOffset::workToBeDone()
+{
+	return !!d;
+}
+
+} // namespace Command