qdoc: Config class keeps track of current path

The Config class is modified to build a single
multimap containing a record for each variable
found in each .qdocconf file. Each reacord
contains not only the name and value of the
variable, but also its location in the qdocconf
file it was read from and the path to that file.
This single multimap replaces 3 maps in the
Config class.

Task-number: QTBUG-30725
Change-Id: I049a69790f943b24c014a24b55b2b39725a1b56f
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>