qdoc: Refactoring of qdoc data structures

qdoc: Refactoring of qdoc data structures

This commit is the beginning of a significant
overhaul of qdoc. A new class, QDocDatabase, is
added, which will eventually encapsulate all the
data structures used by qdoc. In this commit, the
Tree class is made private and only accessible
from QDocDatabase. Several maps structures are
also moved into QDocDatabase from other classes.

Much dead code and unused parameters were removed.
Further simplification will follow.

Change-Id: I237411c50f3ced0d2fc8d3b0fbfdf4e55880f8e9
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@nokia.com>