need to handle case when coqtop hangs.

[aa755]

kill the coqtop process, start a new one and compile to the last successfully compiled offset?
right now; the editor seems to become slightly unresponsive when this unhappens.
cursor movement works, but cannot type anything.
Check if it is possible to handle coqio in a thread which does not mess up with editor's core functions.
Probably, once I was not able to save the file when coqtop hanged -- this is really bad.

right now, coq io is done in a thread created by org.openide.util.RequestProcessor;

[aa755]

after removing some locks, the IDE freeze issue seems to be gone.
(probably, the IDE UI threads were waiting on locks held by the thread
talking to coqtop)

[aa755]

As mentioned in coq-club forum, to interrupt coqtop when it is taking too much time on
a single command, manually send SIGINT to the associated coqtop process. In windows, I have had success w/
http://www.latenighthacking.com/projects/2003/sendSignal/