TilesRendererBase: Cleanup event naming, similar / duplicate events

[gkjohnson]

Especially with tile loading there is:

tiles-load-start
tiles-load-end

tile-download-start
load-content
load-tileset
load-model

These could be cleaned up a bit to normalize on a common structure. The last four especially. "tile-download-start" could be replaced with "load-model-start" and "load-tileset-start" and the last few could be replaced with "load-tileset-end" and "load-model-end". "load-content" seems to not be used anywhere and so we can probably remove (deprecate) it?

[gkjohnson]

From #1408 - one issue is that we can't necessarily know ahead of time whether a tile is "geometry" or a "tileset". Even checking for "json" is not enough since things like "subtree" will still be loaded yet aren't geometry.

[gkjohnson]

"dispose-model" is also present without sibling events for "content", etc

[gkjohnson]

We'll keep these for now until a problem arises. Some events were adjusted in #1601