Further simplification of Tiddler.update()

Don't bother to assign the expression iterated through to a variable
when it is only used by the following "for" statement.

Add to the "actions" list using extend() instead of list concatenation.
This avoids creation of a new list object.
Collect links in a set in Tiddler.update()

Previously, the links were collected in several lists, then combined,
converted to a set (to remove duplicates) and then converted back to
a list. That involved a lot of unnecessary work.
Also deduplicate image references in Tiddler.update()

The links were deduplicated, but the images were not. I don't see any
reason for this difference.