[Gecode] Some minor changes...

[Christian Schulte]

Dear all,

In order to make things more consistent, I did the following:

 - ME_GEN_FALSE, me_false are now called ME_GEN_FAILED, me_failed (to be
consistent with the ExecStatus).
 - The different status types for posting, propagation, committing, etc
has been unified to a single type ExecStatus.
   The macros have been adopted accordingly.
 - BranchDesc is now BranchingDesc (as it does not provide information
about a single branch but an entire node!)
 - Abstractions for common branchings (variable, value selection) have
been added
 - And more stuff.

What I am still thinking about is to rename priority in cost (after all,
it is about cost!): currently BINARY_HI means high cost but low
priority. So also misnomed. Don't know wether I want to change to cost.

I also fixed a major bug in processing the modified set variables: here
the event was always assigned...

Cheers
Christian