Fbs

1. You no longer have to subclass `ApplicationContext` in order to use fbs. Instantiating it is enough. See the [new default `main.py`](https://github.com/mherrmann/fbs/blob/4a29030e2e6d3244dfd3b8aaaa880947e3a42e81/fbs/builtin_commands/project_template/src/main/python/main.py) generated by `startproject`.
2. Previously, fbs always included `boto3` as a dependency. That is, when you did `pip install fbs`, then `boto3` was installed as well. This however is only required when you use fbs's `release` or `upload` commands. This release changes it so `boto3` isn't installed by default. If you do want to use `release`, you need to `pip install fbs[upload]`.
3. fbs now declares PyInstaller as a dependency. So you no longer have to do `pip install fbs PyInstaller==3.4`. It suffices to `pip install fbs`.

* Don't prompt "PyQt5 or Pyside2?" if the user has exactly one of the two installed.
* Automatically set [`QApplication.applicationName()`](https://doc.qt.io/qt-5/qcoreapplication.html#applicationName-prop) from the `app_name` setting. This automatically gives all windows a nice title.

Resolves 96.

1. Add support for PySide2 5.12.2.
2. Ask which Qt bindings to use (`PyQt5` or `Pyside2`) in `startproject`
3. Fix a small TypeError (91).

Thanks to cmaureir for 1. and 2.

apt-get update` on the Ubuntu 14 Docker image no longer works. This means that apps built with fbs now only support Ubuntu 16.04+. That's fair enough, because Ubuntu 14's LTS support ends in April 2019.

Fixes 83.