Add buttons to jump to next/prev outputmark in an outputwidget.

I added buttons to all output widgets instead of the one for the
build tool view; we can easily restrict this to a certain type of
view if we want to.

The only problem with the buttons is that they are enabled even if
there are no outputmarks to jump to. At the time of creating this
diff, I did not see an easy way to find the number of outputmark.

REVIEW: 115600
BUG: 330206