Partial project loading in the editor

**Problem:**
Loading a large project can take a significant amount of time and consume a lot of memory.

**Task:**
Change the editor so it can work on a partially loaded project. Instead of loading the entire project up front, it should load resources into the project graph as needed.

**Prerequisites:**
* We likely need to change how the undo system works, because we need to be able to undo changes to the project graph without going back to a state where a late-loaded resource is no longer loaded.

**Expected outcome:**
* Users don't have to wait for the entire project to load, and could get to work on the parts they need quicker.
* Editor memory usage will decrease significantly, since we don't have to keep the entire project in memory at once.