Qutie

- fixed color issue 7

- fixed `Settings` filename issue 9

- class `Application` now closes all windows properly on `Ctrl+C`
- class `TreeItem` can be constructed without arguments.
- Now depends on PyQt5 < 5.15 as workaround for 7

- enhanced class `Worker` (added event `started`, providing traceback on event `failed`)
- various bug fixes

- enhanced class `Worker`
- added setter for property `current` (classes `Tree` and `Table`)
- various bug fixes

- fixed issue 2