applet: refresh slider value when `to` changes on startup

The value is not updated when the component loading is not complete.

See https://invent.kde.org/qt/qt/qtdeclarative/-/blob/bbeca33068718b8f5b820d20085e42ca86a5ffd5/src/quicktemplates/qquickslider.cpp#L322

BUG: 472245
FIXED-IN: 5.27.9


(cherry picked from commit fb89a7c9)