Piano+LibDSP: Move Track to LibDSP

This is a tangly commit and it fixes all the bugs that a plain move
would have caused (i.e. we need to touch other logic which had wrong
assumptions).

Userland/Applications/Piano/ProcessorParameterWidget/Slider.h

@@ -29,4 +29,6 @@ protected:
 private:
     // Converts based on processor parameter boundaries.
     int linear_to_logarithmic(int linear_value);
+
+    bool m_currently_setting_from_ui { false };
 };