sched: canceling a task did not work apart from some test cases

When explicitely (e.g. manually via the console abort/cross icon)
canceled, tasks were not canceled.
Tests were ok because in test cases the task were canceled by automatic
cancelation on queuing an already queued task (discardaliasesonstart).
ui: adding a skull icon in tasks model max duration before abort column
alerts: stop rules faking notifying cancel and reminder for ui purpose

notifyCancel() and notifyReminder() returning true instead of false
avoids making the ui looks odd
ui: graphviz-powered tasks trigger diagram