Cleanup
Refactor|libgui|libdeng2: Rich text styles

The IStyle interface is now queried for the formatting of the generic
content styles.

Also added an (untested) way to restore the previous range's format
(using stack semantics).
Client|LogWidget: Adjusted log entry styling
libdeng2|libgui|Atlas: Basic thread-safety

Atlas locks itself automatically when the public API is being called.

Also do the same in MemoryLogSink.
libdeng2|Range: Added operator to extend the range to include a value
libdeng2|Asset: Added a new Asset state (Recovering)

Sometimes it's useful to indicate that an asset is presently going
through the process of being recovered into the Ready state.