Allow qdocconf to specify where the examples are located in install.

Allow qdocconf to specify where the examples are located in install.

We have qdoc configurations for the different qtbase libraries and
modules now, which specify e.g. "../../../examples/opengl" for the
exampledirs. That means that qdoc finds examples like
"2dpainting/2dpainting.pro" which is then installed into
"[QT_INSTALL_EXAMPLES]/opengl/2dpainting/2dpainting.pro". (At the moment
even into "[QT_INSTALL_EXAMPLES]/qtbase/opengl/...", which will be
changed to match the structure in source.)

In order for Qt Creator to find the examples, qdoc must write the path
behind [QT_INSTALL_EXAMPLES] into the example manifest file. This patch
introduces "examplesinstallpath" that allows qdoc configs to specify the
missing part, e.g. the opengl qdocconf can add

examplesinstallpath = opengl

to tell qdoc that the examples will be installed into
"[QT_INSTALL_EXAMPLES]/opengl/"

Task-number: QTBUG-27801
Change-Id: Idf518e16d3ca0f9522b084e92f34725b93e9d11f
Reviewed-by: Martin Smith <martin.smith@digia.com>