qdoc: qdoc now can run in 2 passes

qdoc: qdoc now can run in 2 passes

Two command line options have been added, -prepare and -generate.
If you run qdoc with -prepare, qdoc reads and parses the source
files but does not generate the documentation. It only creates
the .index file for the module you are running qdoc on.

If you run qdoc with -generate, qdoc reads and parses the source
files as well as the .index files created by running qdoc with
-prepare, and it generates the documentation but no .index file.

If you run without either option, qdoc runs as before, i.e. it
runs both passes as a single pass.

Task number: QTBUG-27539

Change-Id: Idbfe3f0f9dff58283596b504f00dff3f70f6e371
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>