Move play/pause button

[t-makaro]

When text for the value of the timeline slider is long, the play/pause button can cover it up.

We have a few options:

1. Place the text for the slider below. (Probably not doable without making a custom slider).
2. Move the default button placement below the slider.
3. Make the button placement intelligent.

I am aiming for 2.

Also, it is worth considering adding a keyword argument that will move the button between the current placement and the new placement.

[t-makaro]

Related #20

[TomNicholas]

In #46 I tried to solve this with a 4th option:

4. Make the slider size intelligent.