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.h

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+// Note: this header file is used by the undo-machinery and should not be included elsewhere.
+
+#ifndef COMMAND_PICTURES_H
+#define COMMAND_PICTURES_H
+
+#include "command_base.h"
+
+// We put everything in a namespace, so that we can shorten names without polluting the global namespace
+namespace Command {
+
+class SetPictureOffset final : public Base {
+public:
+	SetPictureOffset(dive *d, const QString &filename, offset_t offset); // Pictures are identified by the unique (dive,filename) pair
+private:
+	dive *d; // null means no work to be done
+	QString filename;
+	offset_t offset;
+
+	void undo() override;
+	void redo() override;
+	bool workToBeDone() override;
+};
+
+} // namespace Command
+#endif