Selection: move commands/command_private.* to core/selection.*

The file command_private.cpp had functions concerning selections
only. To make these functions accessible from outside the undo
machinery, turn it into a part of the core-library. Currently,
only C++ functions are exported. We might think about also
exporting a C interface.

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

core/selection.h

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+// Selection related functions
+
+#ifndef SELECTION_H
+#define SELECTION_H
+
+/*** C++-only functions ***/
+
+#ifdef __cplusplus
+#include <vector>
+
+struct dive;
+
+// 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).
+void setSelection(const std::vector<dive *> &selection, dive *currentDive);
+
+// Get currently selectd dives
+std::vector<dive *> getDiveSelection();
+
+#endif // __cplusplus
+
+#endif // SELECTION_H