Show a cylinder widget in the dive planner

This is intended to allow the user to provide the gases / cylinders that
she will be diving with. With that information the planner can warn the
user about insufficient gases, but more importantly it can show relevant
gases in the gas select drop down.

Right now the add cylinder button doesn't add a cylinder - that's a
problem.

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