user survey

[Dirk Hohndel]

I just pushed a small commit that creates a really ugly user survey dialog.

Some things are implemented. In a tagged build (i.e., a release) the user
will be asked seven (or more) days after they first used Subsurface. In a
development build (version contains a '-') the user is asked the second
time they start Subsurface.

If the user cancels the dialog, they are asked if we should ask again or
just go away.

Besides that there's very little there.
- we do nothing with the answers to the questions
- we don't populate the system description block (waiting for joshwambua)
- we don't submit anything to our server
- we don't have a backend to submit things into

Why did I push this the day before leaving on my trip? We need to figure
out what we really want to ask in that survey - which data is relevant and
interesting for us? So this gives us time for some conversation.
Oh, and maybe someone else will finish the implementation :-)

/D