Doc: Modularize Messaging Framework

[16:06:49] <jpasion> Essentially, this: http://qt-project.org/wiki/Qt5APIDocumentation
[16:07:43] <jpasion> A basic configuration is here: https://codereview.qt-project.org/#change,47109 You need QMAKE_DOCS and qmake will do the rest. It uses QT_INSTALL_DOCS to determine where to output the documentation.

we should probably also consider removing the generated html from the source tree (docs/html, see docs/src/doc.pri too)