Avoid "t" as variable name in StoryFrame.importSource

It was used for "title", "tiddler" and "widget": saving a few letters
is not worth the confusion.
Use sets in StoryFrame.importSource

This is more efficient than doing membership tests on lists.
Avoid catching KeyError

Exception handling applies to an entire block, so it could also catch
exceptions other than the intended one. Also the non-catching code is
easier to read, in my opinion.
Remove widgets by title

This better matches the fact that widgets are now stored in dictionary.
Cleaned up StoryPanel.removeWidgets

Make use of the fact that widgets are now stored in a dictionary.

Also fixed a weird off-by-one error in the reported counts: one was
added to the counts for reasons unknown.
Rename widgets by old title

This better matches the fact that widgets are now stored in dictionary.
Compare to None using "is"/"is not"

This is faster and safer than calling a possibly overloaded
__eq__/__ne__ method.
Various cleanups related to findWidget()

In particular, don't bother looking up the widget object if all we want
to know is whether or not an entry exists for the given title.
Add isURL() utility function

This removes duplicated code. It also avoids hardcoding URL schemes.

Finally, it fixes a bug in StoryFrame.autoBuildStart() where a
generator comprehension was evaluated as a boolean: it should have
been 'if not any(' or 'if all(not' instead of just 'if (not'.
Fix use of non-existing variable 'noDefaultCSS'

This was introduced in 958bdef and found by Pylint. Leon confirmed on
GitHub that 'not blankCSS' was the intended code.
Merge pull request #159 from mthuurne/cleanup

Various cleanups