Feature: Scene Bookmarks

To make it easier to navigate the Spoke viewport, I propose a system of 'scene bookmarking'.

At any point, the user may 'bookmark' the current camera position in the 3d viewport, similar to annotations in Sketchfab. But rather than for purely presentation purposes, these bookmarks help creators jump to different saved viewpoints throughout the scene, reducing the amount of viewport navigation needed while working.

Some design ideas:

• Bookmarks could be represented in 3d as a billboarding icon, perhaps with its name.
• While looking through the viewport camera, the user could press a hotkey(s) to jump to the prev/next bookmark.
• Bookmark icons (3d) may be selected/renamed/duplicated/deleted just like any other item in the scene hierarchy.
• Order of Bookmarks in the scene hierarchy may be reordered-- their order determines the prev/next order when jumping between bookmarks.

Other considerations:

• May want to display a bookmark's transform info in the Properties panel. This could allow editing of the data to make creating arrays of bookmarks via duplication easier (i.e. "Duplicate bookmark, set its X value 10 meters over, repeat...")
• Might end up wanting a parameter for FOV
• This system could end up having some utility in Hubs--perhaps intersecting with the discussions around clickable 'teleport' points since their seems to be a fair degree of overlap feature-wise.

The issue names no files, tests, or entry points. Start by mapping the viewport, scene hierarchy, and Properties panel systems, then clarify the bookmark data model and navigation interactions. The issue does not define completion criteria beyond its design ideas.