Collect plugin jars in dist folder, like before.

Whenever we build a new jar -- or if the jar in dist is changed --
copy it into the dist directory with a proper name.

Change-Id: If05133cf38f1dbe2ce9ba6ace94bf8cf2b466bc1
Reviewed-on: http://gerrit.dmdirc.com/4060
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>