[widgets][dataformateditor] Add parent to QMenu

Issue found when testing with PySide2.

From the documentation, QPushButton::setMenu
does not take ownership of the menu. With PySide2,
the menu is garbage collected but the actions
created from it are not completely destroyed as
it seems. Giving the menu a proper parent ensure
that its lifetime is correctly handled.
4 jobs for 248_fix_button_menu_lifetime in 10 minutes and 6 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
passed #173267
docker-build
build_linux_36

00:10:06

passed #173268
docker-build
build_linux_37

00:08:06

passed #173265
macosx
build_macosx_36

00:09:04

passed #173266
macosx
build_macosx_37

00:09:14