Undo: make adding of planned dive undo-able

Planned dives were still added by directly calling core code.
This could confuse the undo-machinery, leading to crashes.

Instead, use the proper undo-command. The problem is that as
opposed to the other AddDive-commands, planned dives may
belong to a trip. Thus, the interface to the AddDive command
was changed to respect the divetrip field. Make sure that
the other callers reset that field (actually, it should never
be set). Add a comment describing the perhaps surprising
interface (the passed-in dive, usually displayed dive, is
reset).

Moreover, a dive cloned in the planner is not assigned a
new number. Thus, add an argument to the AddDive-command,
which expresses whether a new number should be generated
for the to-be-added dive.

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

desktop-widgets/command_divelist.h

@@ -82,7 +82,7 @@ private:
 
 class AddDive : public DiveListBase {
 public:
-	AddDive(dive *dive, bool autogroup);
+	AddDive(dive *dive, bool autogroup, bool newNumber);
 private:
 	void undoit() override;
 	void redoit() override;