Refactoring: move undo commands to top level

In the future we might want to use undo-commands for mobile as
well (even if not implementing undo).

Therefore, move the undo-command source from desktop-widgets
to their own commands top-level folder.

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

commands/command_private.h

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+// Private definitions for the command-objects
+
+#ifndef COMMAND_PRIVATE_H
+#define COMMAND_PRIVATE_H
+
+#include "core/dive.h"
+
+#include <vector>
+#include <utility>
+#include <QVector>
+
+namespace Command {
+
+// Reset the selection to the dives of the "selection" vector and send the appropriate signals.
+// Set the current dive to "currentDive". "currentDive" must be an element of "selection" (or
+// null if "seletion" is empty). Return true if the selection or current dive changed.
+void setSelection(const std::vector<dive *> &selection, dive *currentDive);
+
+// Get currently selectd dives
+std::vector<dive *> getDiveSelection();
+
+} // namespace Command
+
+#endif // COMMAND_PRIVATE_H