Change how we display a selection's length in the timeline

[julienw]

cc padenot

Currently, we display the selection's length while selecting, but remove it once we're dong selecting. padenot asked that it's always present, because it's otherwise painful in his daily usage. Let's note it's present at the top left, but this is far and disconnected from the selection.

Instead, we could:

• display the selection length in the ruler instead of just below
• leave it there after the selection is done
• remove it from the top left (which could probably simplify some of the code there BTW).

Current behavior:

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