Conv windows position manager plugin

[Mark Doliner]

This plugin is very popular:
https://sourceforge.net/tracker/?func=detail&atid=390395&aid=1414861&group_id=235

It remembers the position (and sizes?) of your conversation windows.  There
might only be demand for it for Windows.  The author, Michael Sartain,
provides the source code and a Widows binary.

We should consider maybe shipping the plugin with Pidgin.  Or rolling the
functionality directly into Pidgin.  It might only make sense for MS Windows.

-Mark