Clear PN2 PO2 PHE GasItem

After the dive log has been closed, the ProfileWidget2 does not clear
the pn2 po2 and ph2 gas line.

This patch cleared the three lines after dive log closed.

Acked-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Hugle <wulong@comp.nus.edu.sg>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Profile2: use unsigned int for the divecomputer number

display.h: dc_number is unsigned int, thus a couple of warnings
may pop-out.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>