Fix bug #8032 (timeout when using the Sweave module)

Fix bug #8032 (timeout when using the Sweave module)
After the timeout elapses, the user is notified that a command is taking
a long time to complete and is given the choice to stop it. If the user
decides to let the command run, the timeout is increased, otherwise the
command is killed. One is prompted a first time after 3 mins, a second
time after 9 mins, a third time after 27 mins, and so on, i.e., the n-th
prompt occurs after 3^n minutes.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/branches/BRANCH_2_0_X@40881 a592a061-630c-0410-9148-cb99ea01b6c8