Let QDoc output to subdirectories

Currently qdoc can only output content to one directory. When using a repository with subdirectories this might make a mess.

For instance you download the qt5 bundle (qt5.git), you will now have subdiretories for all modules, such as qtbase, qtdoic, qtdeclarative. If those modules now contain a file with the same name (say index.qdoc which should generate index.html) they will overwrite each other.

A solution could be that you would output to <outputdir>/qtbase/index.html and <outputdir>/qtdoc/index.html. A problem that would appear here however is that it is unclear to which page you want to link if the title, input and output names are the same (index.qdoc generates index.html with title Index) a way to circumevent that is to set the title of the page correctly to something like QtBase Index.