Moved from bitcoin/bitcoin#11826.

This is a RFC regarding adding a new Activity window in the UI, which allows to:

• have activity history (truncated, etc);
• pause or cancel tasks if supported individually;
• have multiple progresses concurrently.

In terms of patch, ShowProgress() signal is removed everywhere and a RAII class is added along with some UI classes:

class Activity
{
    std::shared_ptr<ActivityData> const d;
public:
    Activity(const std::string& title);
    ~Activity();

    void SetTitle(const std::string& title);
    void SetProgress(int progress);
};

Consider this the minimal interface for an activity. Some other ideas:

• custom progress range (so the caller doesn't have to calculate percentage)
• add something like SetPause(bool), IsPaused();
• also Cancel() and IsCancelled();
• add something like SetSource(string) (for instance SetSource("wallet1")).

The current implementation is thread safe. For now, I've also added a new menu item Window where I think we could add Console too (another discussion thou).

Please see the capture below.