Add icons to the tasks in the task list for more visual distinction among many items

[andergmartins]

We started getting more available tasks in the Checklists page. It seems like it started became a little confuse.
I think if we add icons to each item according to its type we can make easier to distinguish and find each item.

A rough mockup:

I'm still not sure we should display those icons in the checklist metabox (in the post edit pages) because we already have the check/unchecked mark.

[stevejburge]

That's a good idea for the settings. I had been thinking of some sub-headings, but this might be better

[andergmartins]

After implementing #162 we would have more room for the icon