Add ECMSetupQtPluginMacroNames

Summary:
This macro helps with setting up all the boilerplate CMake code needed
to instruct automoc about the custom C++ macros used to define Qt-style
plugin class instances.

Reviewers: Build System, Frameworks, apol

Reviewed By: apol

Subscribers: apol

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D10749