Undo: implement undo of dive site country editing

Simply copy the code of notes editing, but use the taxonomy_* functions
to read and set the value. Moreover, replace the three TAXONOMY_n field
ids by a single TAXONOMY id. We will probably never show one column per
taxonomy field, but rather a single column with a string derived from all
taxonomy fields.

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

desktop-widgets/command_divesite.h

@@ -79,6 +79,19 @@ private:
 	QString value; // Value to be set
 };
 
+
+class EditDiveSiteCountry : public Base {
+public:
+	EditDiveSiteCountry(dive_site *ds, const QString &country);
+private:
+	bool workToBeDone() override;
+	void undo() override;
+	void redo() override;
+
+	dive_site *ds;
+	QString value; // Value to be set
+};
+
 } // namespace Command
 
 #endif // COMMAND_DIVESITE_H