Make formatter configuration dialog resizable

Requested by @quintesse in
https://groups.google.com/d/msg/ceylon-dev/i-AF_HLnR04/4RtXXsgtZa8J

Also removes the minimum size from the individual tab pages - since the
actual content is smaller than 400x400, the scrollbars started to appear
too early when you made the dialog smaller (while everything was still
visible).
Formatter config: fix "show invisible characters"

Previously, when you narrowed the dialog, the checkbox would move
partially out of the dialog instead of staying right-aligned.

(If you make it *very* narrow, it will now displace the "Preview" label,
forcing it to be split up into one character per line, but the proper
way to fix that is probably some minimum sizes.)