UI: better navigation over the log

[theihor]

When the input log is big, navigation is quite inconvenient currently.

You can scroll, but there is no scrollbar visible (because only visible part of the log is rendered). You can also use arrows, pgup/pgdown and home/end.

The "go to line" feature is mostly useless, as user wouldn't know where to jump exactly.

A couple of ideas:

• implement a virtual scroll bar
  • alternatively render entire log if it's small enough
• Replace "go to line" with grep

Content-based navigation would be most useful, but that's a separate issue.

[theihor]

Virtual scroll bar implemented by #17