First implementation of non-UI-locking callbacks.

Sorting plugins no longer freezes the UI. Part of #246.
Added progress dialog.

Early days yet, and for some reason CEF refuses to play the progress
animation if I override the styling. Part of #246.
Implemented custom progress bar.

CEF won't animate the `<progress>` value pseudo-element, though it works
fine in Chrome, so I had to roll my own. Part of #246.
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.
Death to the trailing whitespace!

Sorry about that...