Debounce calls to ProjectManager._renderTree.

This is a possible fix for #9851. In my tests, this can cause a
dramatic speed up when lots of filesystem changes cause the tree
to be continuously rerendered.

I chose 100ms as the debounce time because that seemed quick enough that
users will not be waiting for their actions to be reflected in the tree.
There's already a Jasmine waits(timeout) function. Does that not work here?