Fix missing SASL library in Windows build.

The file libplain-3.dll was not getting included in the installer. A typo in strip exclusion list has also been fixed. This is a residual change for revision 241.
Upstream fix for nss-prefs dll name.

Because of a typo the dll was being named ".dll". This patch has been applied: https://hg.pidgin.im/pidgin/main/rev/1648d789c7d2.
Update the detailed changelog.
Change version from 14.1 to 15.1.