SoundPlayer: Allow playback progress slider jump to cursor

This fix allows us to move the knob wherever we click inside the slider.
The 'jump_to_cursor()' mechanism wasn't working properly because the
player was overwriting the value we had just clicked.

Author: elyse0

Userland/Applications/SoundPlayer/Common.h

@@ -15,7 +15,7 @@ class AutoSlider final : public GUI::Slider {
     Function<void(int)> on_knob_released;
     void set_value(int value)
     {
-        if (!knob_dragging())
+        if (!knob_dragging() && !mouse_is_down())
             GUI::Slider::set_value(value);
     }