Improve timeline/keyframe ruler label display

Right now, the timeline ruler’s labels show up in a rather strange manner as I increase the timeline length. For example, 00:00:05 shows up only when the ruler width is over 7 seconds and a half.

Also, a label disappears immediately after the tick reaches the left edge of the timeline as I scroll horizontally.

Here is my pull request to make the rulers visible a bit more, both in the Timeline and Keyframes panels.