Implemented: renamed the directory specialpurpose to plugins
(OFBIZ-7972)

In another step towards completing the plugin system for OFBiz, we renamed
the /specialpurpose directory to /plugins and changed all occurences of the
word "specialpurpose" to "plugins" in all files found in the system

Reference discussion: http://markmail.org/message/hpyuxkmftiyn44w2


git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1778271 13f79535-47bb-0310-9956-ffa450edef68