Add dive tags and support invalid dives

This started out as a way to keep dives in the dive list but being able to
mark them as 'invalid' so they wouldn't be visible (with an option to
disable that feature).

Now it supports an (at this point, fixed) set of tags that can be assigned
to a dive with 'invalid' being just one of them (but one that is special
as it gets some additional support for hiding such dive and marking dives
as (in)valid from the divelist).

[Dirk Hohndel: merged with the latest code and minor changes for coding
	       style and consistency. Ensure divelist is marked as
	       modified when changing 'invalid' tag]

Signed-Off-By: Jozef Ivanecký (dodo.sk@gmail.com)
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Store the tag names instead of an opaque number

And as we need the names for that, simplify the way we show the tags in the
Dive Info tab (and mark them for translation while we are at it).

In the process I renamed the constants to DTAG_ from DTYPE_ (and made
their nature as being just bits more obvious).

Also mark the box on the Info tab "Dive Tags", not "Dive Type".

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