UI for importing keys

#25
Key import is now working

Refactored GPG service - simplified and sped-up the internals.

Better error detection in GPGWorker service.

Logging service is now more efficient (using noop() to skip log level checking).

RuntimeError.toString() now pretty prints its root causes.

Much easier to sub-class RuntimeError and its descendants.

#25
Merge branch 'feature/25-import-key' into develop