Masterlist update is now non-UI-blocking.

For #246.
Fixed sorting skipping repeated plugin loading.

While it does lead to almost instantaneous sorting, the plugins may have
changed since they were last loaded, and there's no way to tell
currently, so load them all to be safe.
Getting game data no longer blocks UI.

Progress dialogs added for initialisation, game change and window
refocus refresh. Part of #246.