Symbols of detail::View class are public

[adlarkin]

When working on #752, it became apparent that the detail::View class has public visibility. This makes it difficult to change things in this class since it will break ABI. Considering the fact that detail classes were never intended to be used by external users, we should probably make the visibility of this class hidden. I believe that this could be as simple as switching detail::View's visibility to IGNITION_GAZEBO_HIDDEN for Fortress, but as @iche033 pointed out, we will need to test this to ensure that other platforms and libraries aren't broken by this change.

@chapulina, what do you think about this? Are there any reasons why we should keep the visibility of detail::View (and anything else in detail) public, or are we okay to make the change to hidden?

[chapulina]

Ahh that's unfortunate. We should hide everything inside detail.

[adlarkin]

I'm closing this since #788 was merged (it should have closed automatically since this issue was linked to #788, but for some reason, it didn't 🤷‍♂️)