Add 'tasks.onDidEndTask' Plug-in API

[RomanNikitenko]

Add ability to get event on plugin side when a task is terminated
Example:

theia.tasks.onDidEndTask(event => {
      const task = event.execution.task;

      console.log(`The task ${task.name} was terminated`);
});

Signed-off-by: Roman Nikitenko rnikiten@redhat.com

[RomanNikitenko]

@benoitf I rebased my branch and resolved conflicts after your approval.
Could you take a look - is it OK?
The changes was in changelog file only (resolving conflicts).
Thank you!