Edit time dialog gets "End now" and "OK" buttons. for running (no end-time) tasks

[linuxcaffe]

EDIT: simplified

"End now" should be an option, when changing a running task-time,

  [ End now ]   [    OK    ]

OK to change the start-time, without stopping the task

or End now immediately stopping the running task, creating an end-time,which can then be modified, before hitting the usual (big) "OK" button.