clean up handling of LOADING state

Some downstream changes in Endless caused us to switch modes before LOADING was completed, causing terrifying races, empty categories/overviews and even some deadlocked plugins. In the process I found that the handling of the LOADING state was a bit fragile and had regressed upstream. Here are my cleanup patches to make the state transitions into and out of the LOADING state a bit more robust so that both shell and application have a coherent idea of when we're loading or not, and the "loaded" signal is always emitted once when the initial refresh is completed.