SpinBox: omit group separators

Task-number: QTBUG-31839
Change-Id: I90879cbc116f399f07e2ae53e998253f2e81a5e2
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Keep TextField placeholder text visible when focused

This is useful when the widget is focused by default, and also gives
another opportunity to the user to read the message before typing,
removing the need for workarounds like clicking in another text entry to
bring back the placeholder message.

Same as qtbase commit eeb31ad1 for QLineEdit.

Change-Id: I466dd66490e05503b51e208beefd5366b25fd939
Reviewed-by: Caroline Chao <caroline.chao@digia.com>