Add toggel for hover over images and videos

[Mletter1]

Allow the user to control the hover -> display behavior.  In “exploration mode” the current behavior is followed in “presentation mode” the user controls when media is displayed (for example right-click -> select from a set of available media for that point)

[pjcross]

I think the easiest way to address this is to make the first Media selection choice to be "none". That way the default behavior is not to pull images/videos until the user signals their desire for image/video retrieval by selecting a media variable.

[alexsielicki]

I added the "None" choice as the first option in the "Media Set" drop down. Also, I'm disabling the "Pin" choice in the "Selection Action" dropdown when "Media Set" is set to "None".

[alexsielicki]

Defaulting "Media Set" to "None" upon model load when no bookmark is present with Media Set saved in it. @pjcross, is this correct behavior? Or would you prefer the default to be the first available media set?

[pjcross]

Yes. Defaulting to "None" is the correct behavior when no bookmark is present. Disabling "Pin" is a good catch, since you wouldn't be able to pull and pin when the selection is none.