Selecting timeline range should be possible with a keyboard

**Steps to reproduce:**

Although tabbing through timeline tabs is now possible, selecting timeline range isn't possible with a keyboard.

**Expected behavior:**

I should be able to select the timeline using my keyboard.

**Actual behavior:**

Timeline selection is only possible with a mouse/touchpad.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-106)