Fix for #15 and #13

[Juravenator]

These 2 commits fix #15 and #13. More specifically:

• The progress calculation logic has been replaced with a scrollTop-based approach, to account for removed calls
  • This logic incidentally makes the progress 100% on small files that don't have a scrollbar
• The nyancat-view is hidden when no file tabs are opened
• The nyancat-view creates the DOM only once and updates it, rather than destroy and recreate on every scroll event
• The nyancat-view takes all available remaining space in the left status bar
• Replaced the event listener so it also detects scrolling events when the open tab isn't selected (preview tabs)
• Adjusted the css slightly to fix some unwanted padding