Update tiles automatically when material changes

[lilleyse]

Tileset materials were added in #406 but there are some limitations. When you edit a tileset material the changes aren't propagated back to the tiles. There are two types of edits that we need to handle:

• Changing individual material values - this could be implemented by watching for USD notifications and updating Fabric prims (a bit like Imagery layer alpha control #498).
• Changes to the node graph - this is more involved and probably requires USD notification + tileset reload due to the fact that every tile's material is a copy of the material being edited.

[lilleyse]

Alternatively, we could detect when a material is changed and prompt the user to reload the tileset rather than doing it automatically, kind of like the "Fetch USD changes" popup that appears when you edit a USD file. That might be less frustrating.