Remove font customizations

* Should respect whatever font the user has set.
  (“Respect the user’s settings by always using the system font, sizes, and colors.” http://msdn.microsoft.com/en-us/library/windows/desktop/aa511282.aspx)
* Courier looks odd and thin (a11y issue #4082).

src/widgets/trackslider.cpp

@@ -38,10 +38,6 @@ TrackSlider::TrackSlider(QWidget* parent)
 {
   ui_->setupUi(this);
 
-  QFont font("Courier");
-  ui_->elapsed->setFont(font);
-  ui_->remaining->setFont(font);
-
   UpdateLabelWidth();
 
   // Load settings