Better behavior when left-clicking a TCO (#4290)

Instead of calling MouseMoveEvent(), the TCO's "text float" text and position are updated.
This prevents left-clicking the right edge of a resizable TCO from decreasing its size.

Also, removed an unused variable: m_oldTime

include/Track.h

@@ -290,8 +290,6 @@ private:
 
 	TextFloat * m_hint;
 
-	MidiTime m_oldTime;// used for undo/redo while mouse-button is pressed
-
 // qproperty fields
 	QColor m_mutedColor;
 	QColor m_mutedBackgroundColor;