Optimize screen viewport (scroll) when setting cursor

[hedyhli]

This applies to:

• highlighting hovered item (for outline)
• goto location (for code)

General algorithm:

• If total no of lines in buffer is less than available window height, ensure top line is visible at the top of window
• Else if the distance from desired line to set, and last line in buffer, is less than half of window height, ensure last line is at the very bottom of window
• Otherwise, zz

[hedyhli]

This was implemented by simply running normal! zz on every goto_location (configurable via outline_window.center_on_jump). Does not seem to be needed for the outline as of now.