non-focusable button, to avoid second interpretation of SPACE in C+e SPACE (isabelle.set-node-required);

more thorough read_pid with extra delay, to give external process a chance to write pid file before attempting to terminate it -- especially relevant for PIDE document processing, where interrupts can happen in the range of milliseconds;

Added tag Isabelle2013-2-RC2 for changeset 99b9249b3e05

more official task context via Task_Queue.enroll, which is required to participate in group cancellation (e.g. to terminate command exec); tuned signature;