Viewer: Allow environment texture and skybox to be set separately (and a couple other small changes)

[ryantrem]

The main change in this PR is to allow the environment texture and the skybox to be configured separately (this has come up multiple times, including in the forum. To try to keep this simple, Viewer.loadEnvironment now has an options object where you can specify whether the "environment" should be used for lighting or skybox or both. On the html element level, the environment attribute behaves the same as before (sets both the env texture and the skybox), but there are also now environment-lighting and environment-skybox attributes if you want to set them separately. This makes environment essentially a shorthand, somewhat like padding relative to padding-top, padding-bottom, etc.

The other changes included in this PR are:

• Adding a reset camera button (so that discovering double click/tap is not required). This change was based on feedback from @PatrickRyanMS.
• If animation is playing, pause it when activating a hotspot (e.g. moving the camera to a pose associated with the hotspot). Without this, a surface type hotspot will almost certainly have moved by the time the camera finishes its interpolation. This change was based on feedback from @PatrickRyanMS.
• Update the icons to use svg from https://iconcloud.design. This change was based on feedback from @PatrickRyanMS.
• Mark various internal members as @internal so they don't show up in the API docs.

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