Modularization of examples has resulted in putting
all documentation and code for an example in each
example subdirectory. In particular, each example
subdirectory now contains a doc/images subdirectoy,
where images are stored that are linked to from the
example .qdoc files, which are in the doc/src
subdirectory of the example directory.

Because there are so many example subdirectories,
it would be difficult to list all the doc/images
directories in the imagedirs variable of the
qdocconf file. Therefore, qdoc now searches all
the directories listed in the exampledirs variable
to find all the doc/images subdirectories there,
and it adds these to the imagedirs variable for
use in finding images called out in \image and
\inlineimage commands.

Task Nr: QTBUG-27248

Change-Id: I070ba1a558ab32e1db06429a71c083b55f9dd0ea
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>