Version 2.18 - dialog fix and little cleanups

[Eric, just a guy]

Version 2.18 is up. Mainly, I wanted to put in a bug fix for the
"scale" feature, as it was wedged and is needed for the contest.

The whole release blurb: Output block size in statistics popup. Warn
user if unknown block type is encountered. Add a "head block"
placeholder (pumpkin) for now; it turns out to be hard to really add
the new beta's head blocks. Bug fixes: fix the scale dialog (the radio
boxes got separated from one another). Turn default options (connect
blocks, delete floaters) back on when "lesser blocks" option is turned
off. Attempt to find Minecraft worlds a bit better on the Mac (no
promises...). Remove warning about floaters when "lesser blocks" is
used.

In the meantime, for the Curse/Shapeways contest I made two pseudo-
entries (the models aren't my work), to test the system. All entries
to the contest are here, http://www.shapeways.com/search/tag?tag=Curse-Contest_MoP
(just my tests, so far). They're actually more fun to look at on
Sculpteo, http://www.sculpteo.com/en/design/temple-of-the-moon-simple/6MbhZDPq
and http://www.sculpteo.com/en/design/galidin-square-arena/oKaaX3Eg