tags and multi dive edit

[Dirk Hohndel]

I thought I had fixed this a while ago but I just had made it a little
less broken... 
This morning I tried to figure out what semantics SHOULD be and then
implemented just that.

So you select multiple dives that have potentiall different tags. What
does it mean if you then edit the one dive that is shown? I figured the
most intuitive behavior would be this:
- if the user adds a tag to the existing tags of the displayed dive, then
  that tag should be added to the tags of all the selected dives
- if the user removes a tag from the existing tags of the displayed dive,
  then likelways that tag should be removed (if present) from the tags of
  all other selected dives

Two requests:
a) is that indeed the correct semantic for this operation?
b) could you test that what I implemented in master actually does this?

Thanks

/D

PS: daily builds are up for those who can't build from source...