Undo: make adding of weights an undoable action

Introduce an AddWeight undo command. This is modelled after the
numerous dive-edit undo commands. The redo and undo actions are
connected to the WeightModel via two new signals, weightAdded
and weightRemoved.

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

commands/command_edit.h

@@ -330,6 +330,15 @@ private:
 	bool workToBeDone() override;
 };
 
+class AddWeight : public EditDivesBase {
+public:
+	AddWeight(bool currentDiveOnly);
+private:
+	void undo() override;
+	void redo() override;
+	bool workToBeDone() override;
+};
+
 } // namespace Command
 
 #endif