Fix a long standing bug when editing dives

Calling edit from the context menu creates a combined editing widget that
contains both dive info and equipment. When editing cylinders or
weightsystems from that widget and confirming those edits with OK those
changes were already committed to the current_dive - regardless on which
dive the user clicked. Worse, even when the user clicked Cancel in the
edit widget, any changes to the equipment stayed in effect.

This had especially confusing consequences when editing multiple dives.

As a workaround this commit adds a global edit_dive variable. This fake
dive is edited by the secondary editing widgets and if the user accepts
changes with OK then they are copied over to the current dive (or all
selected dives in multi dive editing mode).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

dive.h

@@ -262,6 +262,10 @@ struct dive {
 	struct sample sample[];
 };
 
+/* this is a global spot for a temporary dive structure that we use to
+ * be able to edit a dive without unintended side effects */
+extern struct dive edit_dive;
+
 extern GList *dive_trip_list;
 extern gboolean autogroup;
 /* random threashold: three days without diving -> new trip