State Transaction Machine

The process_transaction function adds a new edge to the Dag without
executing the transaction (when possible).

The observe id function runs the transactions necessary to reach to the
state id.  Transaction being on a merged branch are not executed but
stored into a future.

The finish function calls observe on the tip of the current branch.

Imperative modifications to the environment made by some tactics are
now explicitly declared by the tactic and modeled as let-in/beta-redexes
at the root of the proof term. An example is the abstract tactic.

This is the work described in the Coq Workshop 2012 paper.
Manual fixed w.r.t. STM
Fix testsuite so that it works with STM
Coqide ported to STM

Main changes for STM:
1) protocol changed to carry edit/state ids
2) colouring reflects the actual status of every span (evaluated or not)
3) button to force the evaluation of the whole buffer
4) cmd_stack and backtracking completely changed to use state numbers
   instead of counting sentences
Support Proof General
Stm debug prints using threads to be fast
Simple machinery to detect EXTEND that interpret during parsing
Vernac classification streamlined (handles VERNAC EXTEND)

The warning output by vernacextend when the classifier is missing
is the documentation of this commit:

Warning: Vernac entry "Foo" misses a classifier. A classifier is a
function that returns an expression of type vernac_classification (see
Vernacexpr). You can:
- Use '... EXTEND Foo CLASSIFIED AS QUERY ...' if the new
  vernacular command does not alter the system state;
- Use '... EXTEND Foo CLASSIFIED AS SIDEFF ...' if the new
  vernacular command alters the system state but not the parser nor it starts
  a proof or ends one;
- Use '... EXTEND Foo CLASSIFIED BY f ...' to specify a global
  function f.  The function f will be called passing "Foo" as the
  only argument;
- Add a specific classifier in each clause using the syntax:
  '[...] => [ f ] -> [...]'.
Specific classifiers have precedence over global classifiers. Only one
classifier is called.
Test for Paral-ITP