plugins: add ChatHandler API

src/api/pluginmodel.h

src/chatview.cpp

src/pluginmodel.cpp

src/qtwrapper/pluginmanager.cpp

src/qtwrapper/pluginmanagerMock.cpp

src/qtwrapper/pluginmanager_wrap.h

src/web-chatview/chatview.html

src/web-chatview/chatview.js