Dive sites: prepare for dive site ref-counting

Add a dive site table to each dive site to keep track of dives
that have been added to a dive site. Add two functions to add
dives to / remove dives from dive sites.

Since dive sites now contain a dive table, the order of includes
had to be changed: "divesite.h" now includes "dive.h" and not
vice-versa. This caused some include churn.

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

core/dive.c

@@ -10,6 +10,7 @@
 #include "libdivecomputer.h"
 #include "device.h"
 #include "divelist.h"
+#include "divesite.h"
 #include "qthelper.h"
 #include "metadata.h"
 #include "membuffer.h"