Slider is not set if only clicked

In the current implementation, a slider is not set to a position if it is only clicked. However, on Android native toolkit and in common expectation of users, clicking (but not dragging) somewhere in a slider should set the slider to that value.

This behaviour is easy to correct, in `QQuickSlider::mouseReleaseEvent(...)` in qquickslider.cpp, the test `if (keepMouseGrab())` at line 519 should be removed.

Without changing qquickslider.cpp, the current behaviour is not easy to correct, especially because the C++ API of Qt Quick Controls 2 is marked as private, so sub-classing the control is not future-proof.

While this issue could be considered a wish, I believe it is an important enough unexpected behaviour of UI to classify as a bug.