Make sure the correct position for the plugin is used when showing

When the plugin was windowed then it was always being shown at 0x0 of
the containing window rather than the correct position of the plugin
inside the containing window. This enesures it is placed correctly in
that case.

Task-number: QTBUG-31730
             QTBUG-33053
             QTBUG-32546

Change-Id: I5ce0321924d288437d483002e9cb6dd1c98b6863
Reviewed-by: Michael Bruning <michael.bruning@digia.com>