Always keep the cursor visible

When the cursor goes past the viewport, it becomes invisible. The viewport should be shifted in order to make it visible again.

This should work when:

• the cursor goes past the left border of the viewport via the Left key press
• the cursor goes past the right border of the viewport via the Right key press
• the cursor goes outside the viewport when dragged by the mouse
• the time pane is zoomed in
• the horizontal scroll bar changes value