Feature wish: avoid reseting the windows layout as much as possible.

[Matafou]

I.e. avoid calling proof-layout-windows when not absolutely necessary. In particular when switching the scripting buffer (which restarts the coq process) we should avoid destroying the windows sizing and splitting the user made, it is annoying.

The user can always call proof-layout-windows by hand (C-c C-l or menu of M-x proof-layout-windows) anyway.

Actually it implies two modifications:

• avoid calling proof-layout-windows unless necessary
• avoid killing the associated buffers, which has been already implemented by this PR: #639. We need to set proof-shell-kill-function-also-kills-associated-buffers to nil by default