non-commit: abiword-plugins: rewrite url plug-ins

A mixture of extensions, clean-up & fixes. I'd like to commit, but it's
all so horribly confusing at the moment!

AbiURLDict: Add Webster, Cambridge and, er, Oxford (okay, yes, I know,
but it works for me, and I'm want to add some configuration mechanism
for this kind of thing anyway)

AbiGoogle: Add Yahoo

AbiWikipedia: currently looks up English wiki, but could be worth
making this language-sensitive.

Sulking,
Frank