Add scrollbar to lights

[elgopher]

Is your feature request related to a problem? Please describe.
When there is more than ~1400 tests there is no space to draw all test result lights. It could be helpful to show scrollbar in such case and let user scroll over all tests.

Describe the solution you'd like
Add vertical scrollbar. The new implementation should be fast - drawing 1400 lights on screen is cpu-intensive task. And drawing even more could be very innefficient. The new implementation should draw only visible lights on the screen.

Describe alternatives you've considered
Use test results tree instead.