Add depth colum to cylinder model

To make the planner work this adds a new column to the Cylinder widget
(depth - for the depth at which we want to change to a certain gas
during deco).

This also tries to hide that column in the equipment view and hide the
start/end pressure columns in the planner view. Oddly that fails :-(

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

dive.h

@@ -143,6 +143,7 @@ typedef struct {
 	cylinder_type_t type;
 	struct gasmix gasmix;
 	pressure_t start, end, sample_start, sample_end;
+	depth_t depth;
 } cylinder_t;
 
 typedef struct {