#179 Add status bar

Adds status bar. This allows to show current number of jobs in queues, transient messages, and makes it possible to extend it in future to track database interaction failures (see related issues). In particular, we will be able to show warning on status bar and avoid entry of the data if some database constraint will fail. However, to properly handle such warning, some better tracking of saved state should be added and it is out of scope for this MR.

In few commits, thread pool was somewhat changed to make it to follow inheritance better and allow to track number of jobs in queues.

Please review

Fixes #179 (closed) Related to #156 (closed) #155 (closed) #169 (closed)