Add a way to manually trigger update

Add a way to manually trigger an update of the project storage updater.

In the Asset Importer, we generate QML files, which can depend on each other.
In this case, we want to notify the project storage that a particular QML file was changed/created and have a synchronous update.

We can also update a directory. Any asynchronous compression/timer has to be bypassed.

The effect should be as if the file watcher was triggered but synchronous.