Integrate running QDoc in single-exec mode into the build system

QDoc in 5.5 will support the single-exec mode, which generates documentation for multiple modules in a single run. This speeds up build time for documentation considerably.

Implement an easy way to build docs using the new mode. This involves having a qmake target(s) similar to 'docs'/'html_docs', and a way to maintain the list of doc modules that are part of Qt.