qmake vcxproj generator: write PrimaryOutputExtension value

qmake vcxproj generator: write PrimaryOutputExtension value

Second attempt. MSVCPROJ_TARGET contains the resolved target name,
including version number and target extension.
We're splitting this value into PrimaryOutput and
PrimaryOutputExtension.
PrimaryOutputExtension is only written if it contains a non-default
value.

Task-number: QTBUG-26782
Change-Id: I4b828dc5dd47322f653585aee1a5767f0cf8bd48
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
(cherry picked from commit qtbase/d1e950b919615c1ae754e07899f1b45303c17654)
Reviewed-by: Andy Shaw <andy.shaw@digia.com>