Removed unused imports.
Another unused import.
Changed widget name (as shown in the title) to the code.

This way is easier to override it by subclasses.
Moved translations to proper new place.
New widget for possible next executions.

The poll trigger, for example, could be scheduled but it
may or may not launch a real execution. This widget would
show the next time the polling will run.
Fixed typo
Translated string (es).
Optionally pass a Trigger class as an argument.

The schedule will be checked for that particular
class.

So the code can be reused for SCMTrigger, for example.
Using a class variable to set the Triger class that will
be checked.

In this Widget it will be the TimerTrigger. A subclass will
set a different Class for overriding this behavior.
This Widget will show schedules for SCMTrigger (polling schedules).
Added configuration option to decide whether the Possible Next
Executions Widget should be shown or not.

By default don't show it.
Merge remote-tracking branch 'origin_gh/master'