Fix QDir::operator==(const QDir &) const

Fix QDir::operator==(const QDir &) const

We can't rely on absolute paths when comparing directories for equality
as these don't take into account symbolic links and may also bypass ../
and ./ simplification.

Instead, canonical paths must be computed and can then be compared
according to the case sensitivity rules for the platform or file engine,
as is done in QFileInfo.

Task-number: QTBUG-20495
Reviewed-by: Prasanth Ullattil
(cherry-picked from dcee6e1371d899eb79717b8e3f3eec08b765db82)

Change-Id: Ib5f2a6ee11311c55782ea5dd0e9c3b45f9231686
Reviewed-on: http://codereview.qt-project.org/5812
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>