Open interactive area if chapters or references are present in video

[Splines]

This fixes #638.

For reviewers

In the code, probably due to historic reasons, references are referred to by "metadata" while chapters are called under the name "chapters".

• Interactive area should be closed if neither chapters nor references are present (1 case)
• Interactive area should be opened if either chapters or references are present (3 possible cases)
• Interactive area itself should still work as usual, e.g. users should be able to open/close it by pressing the button, or by using the keyboard shortcut i. The interactive area should display the elements just as was before.

Note that the interactive area might be opened even though no element is shown. This is by design: references can pop up at a later point in time in the video. If any reference is present (no matter the timestamp), the interactive area will already be opened for users to take note that a reference just appeared.