Minor refactor for creation of sub-windows

This refactors the sub-window code a bit so that instead of deleting the
window pointers, it calls QWidget::close() on them to safely trigger a
normal close on them instead (which will also delete them).
Moves setting the DeleteOnClose flag from inside of the Dialog classes
into the OBSBasic class, to make that behaviour more obvious.

obs/window-basic-transform.cpp

@@ -37,8 +37,6 @@ OBSBasicTransform::OBSBasicTransform(OBSBasic *parent)
 	  ui      (new Ui::OBSBasicTransform),
 	  main    (parent)
 {
-	setAttribute(Qt::WA_DeleteOnClose);
-
 	ui->setupUi(this);
 
 	HookWidget(ui->positionX,    DSCROLL_CHANGED, SLOT(OnControlChanged()));