Add custom Cesium3DTileset Inspector

[j9liu]

This should be merged after #76. I noticed while changing the ion asset ID / access token in the Cesium3DTileset's inspector that it would trigger OnValidate() after every single character change, which was annoying when I was typing a longer asset ID into the field. Since this needed to be done anyway for some of the options (i.e. disabling irrelevant fields depending on toggled options), I went ahead and created a custom version of the inspector.

It should appear the same as the default inspector, but now there are two buttons at the top: Refresh Tileset and Troubleshoot Token. The ion Asset ID and ion Access Token fields don't process the user's changes until they have pressed enter / clicked outside of the text field. Additionally, changing some enums / toggles disables other options. For example, selecting From Url as a tileset's data source disables the ion fields.

[j9liu]

While comparing the UI with the one in Unreal Engine, I realized we forgot an option for showCreditsOnScreen. I added that just now, though it's rendering incorrectly:

I'll debug this in a later PR.

[j9liu]

While comparing the UI with the one in Unreal Engine, I realized we forgot an option for showCreditsOnScreen. I added that just now, though it's rendering incorrectly:

I'll debug this in a later PR.

Turns out this is exactly how it appears in Cesium for Unreal, so it seems like something would have to be adjusted in cesium-native.

In particular, I'm not sure why the delimiter • is counted as credit, both in cesium-native and in CesiumJS 🤔