WIP: Replace loading notification with a spinner

This is mostly ready, but it doesn't make sense to merge it now, since the collection loading is so slow.

Remaining things:

• Crossfade transition currently employs a hack. This will need to be fixed. Alternatively, we can do crossfade for collection from/to game transition as well, in that case the hack will not be needed.
• This exposes #98 (closed) particularly badly, as it happens 90% times with Games view right after loading.
• Test how it interacts with empty state.
• Test this with opening URIs directly.

This will allow us to fiх #105 (moved) cleanly later.

Screencast:

Screencast_from_05-09-18_20_36_45