Fbs

It is now even easier to define custom exception handlers. See the source code documentation in the module.

fbs uses a [custom `excepthook`](https://github.com/mherrmann/fbs/blob/1ad32d133a6163e7a76707820936f6b1de54569d/fbs_runtime/excepthook.py) to fix some shortcomings of both Python and PyQt5/PySide2. Previously, it was hard to subclass fbs's classes that do this. In particular, subclasses had to call a private method (`_add_missing_frames`) to get the full stack trace "added" by fbs's custom Excepthooks. Now, `QtExcepthook#handle(...)` gives you the full stack trace without you having to call private methods.

This new module lets you very easily implement a license key functionality for your app. Please see the associated [documentation](https://build-system.fman.io/manual/#license-keys).

See `ApplicationContextexcepthook`.

Also add an example of this to the default `main.py` that gets generated when you do `fbs startproject`.

See 67.