debian: don't split out libmypaint* (UNREL)

Convinced myself that it's probably a bad idea to attempt to split out
libmypaint: the ABI is still rather slushy, upstream don't build shared
libs by default yet, and I don't want to place unnecessary binNMU burdens
on any packages building against it.

https://mail.gna.org/public/mypaint-discuss/2013-01/msg00021.html

Probably doesn't make sense to split out the brushes either.

Reverts 0ad271a.
debian/changelog: formatting fixes, mention close

Mention closing http://bugs.debian.org/660881 as requested by Axel Beckert.
Thanks, Alex!

Stylistic: settle on a style for (Closes: ...).

Hopefully this sort of retroactive change is OK.
debian/changelog: convey bug closures in aggregate