Add "Apply"/"Cancel" buttons to dive equipment page

Add "Apply"/"Cancel" buttons to dive equipment page

It's too damn easy to make mistakes and not even notice them (odd gtk
widget selection and keyboard input), or just start editing a cylinder
thing and realize it was wrong.

So instead of always saving the equipment information implicitly, add
explicit "Apply" and "Cancel" buttons that save the information (or
re-load it from the dive data structure)

So now you need to press an extra button for your changes to *really*
take effect.  It can be a bit annoying, but it's better than the silent
accidental equipment change that could happen before.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>