Add better highlighting for horizontal timeline events

[seanlowe]

Release: v2.1.7

Changes:

• added event listeners onto timeline events to run some logic when items are being hovered on
  • adds a custom CSS class .runtime-hover on mouse enter, and removes it when the mouse leaves the event
  • adds some logic to set a CSS variable with the particular event's background color
• added styling rules for CSS class .runtime-hover that takes into account the current background color (that was pushed into a CSS variable by the event listeners)
• allowed for timeline event objects to have a copy of the data from the user-created events (HTML/FM)

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Resolves #34