Make Scheduler watch job widgets changes properly, so that setDirty is
called appropriately. Move job edition checks before job creation to
simplify the algorithm. Clarify when add button enables.

Fix wrong QT slot connections on QDoubleSpinBox.

Job edition in the Scheduler tab.

Jobs should properly reevaluate as they are edited, as the original
algorithm was intending to. Part of the code I removed during the past
months incorrectly assumed that there was support for canceling changes
made to a SchedulerJob.

If that feature is needed, it is relatively simple to clone a backup of
the SchedulerJob when it is opened for editing, and restore that backup
when clicking on a cancel button. However, said cancel button should not
be at the same location as the job removal button for clarity of interface.