qdoc: Add option to drop detailed location info from .index and .webxml files

For each item (node), QDoc writes the absolute path/line number of the declaration into the .index. These appear also in .webxml output. Aside for debugging purposes (and maybe reporting specific cross-module documentation warnings) this information is not useful and it complicates testing of QDoc's output.

Introduce a config variable that can be used for disabling this info.