Fix memory leak

Set proper parent to item delegate.

9801== 656 (16 direct, 640 indirect) bytes in 1 blocks are definitely lost in loss record 4,386 of 4,828

9801== at 0x4C29216: operator new(unsigned long) (vg_replace_malloc.c:334)

9801== by 0x46543D02: KisGmicWidget::createMainLayout() (kis_gmic_widget.cpp:87)

9801== by 0x465442A2: KisGmicWidget::KisGmicWidget(KisGmicFilterModel*, QString const&) (kis_gmic_widget.cpp:58)

9801== by 0x46523988: KisGmicPlugin::slotShowGmicDialog() (kis_gmic_plugin.cpp:218)