Introduce a new QtDoubleSpinBox subclass for the
stock QDoubleSpinBox and use it for the native Qt
weld::SpinButton implementation.

Initially, the new subclass doesn't add any
additional functionality, but that will be
added in future commits to provide functionality
needed to implement more of the QtInstanceSpinButton
methods.

Change-Id: Icebbf6485172b065d6a6d2c1c8ef87be9d3d244e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/177435
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>