config: generate sample-config files containing all and only classes relevant to configurable traits

[ankostis]

The motivation is the user to know the relationships between the classes
in the generated sample configuration-file, so that he can choose at
which inheritance-level to override some trait value.

[ankostis]

Rational: This PR ensures that the sample-config file will describe to the user AND contain all the classes on which a trait-value may be overridden:

• either on the class owning the trait,
• or on its subclasses, even if those subclasses do not define any traits themselves.

[ankostis]

Attaching generated configs to facilitate review.

The differences between the 2 listings are the following:

1. The NoTraits class is included in the sample config-file (at the bottom),
   although it has not traits of its own. That way, the user may add overriding values for traits present in its parent configurables (Foo and `Bar).
2. The parent configurables of each class are listed in the parenthesis after its name.
3. The identation of the property-description is different from the value-overriding python-statement, visualized best in the Foo.i property which has a long help-text.

With this PR:

#------------------------------------------------------------------------------
# Application(SingletonConfigurable) configuration
#------------------------------------------------------------------------------

# This is an application.

## The date format used by logging formatters for %(asctime)s
#c.Application.log_datefmt = '%Y-%m-%d %H:%M:%S'

## The Logging format template
#c.Application.log_format = '[%(name)s]%(highlevel)s %(message)s'

## Set the log level by value or name.
#c.Application.log_level = 30

#------------------------------------------------------------------------------
# MyApp(Application) configuration
#------------------------------------------------------------------------------

# This is an application.

## Load this config file
#c.MyApp.config_file = ''

## Is the app running?
#c.MyApp.running = False

## Should print a warning if `MyApp.warn-typo=...` command is passed
#c.MyApp.warn_tpyo = 'yes the name is wrong on purpose'

#------------------------------------------------------------------------------
# Bar(Configurable) configuration
#------------------------------------------------------------------------------

## The integer b.
#c.Bar.b = 0

## Enable bar.
#c.Bar.enabled = True

#------------------------------------------------------------------------------
# Foo(Configurable) configuration
#------------------------------------------------------------------------------

## Some long running text: Lorem ipsum dolor sit amet, consectetur adipiscing
#  elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut
#  enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut
#  aliquip ex ea commodo consequat.
#c.Foo.i = 0

## The integer j.
#c.Foo.j = 1

## First name.
#c.Foo.name = 'Brian'

#------------------------------------------------------------------------------
# NoTraits(Foo,Bar) configuration
#------------------------------------------------------------------------------

Without this PR:

#------------------------------------------------------------------------------
# Application configuration
#------------------------------------------------------------------------------

# This is an application.

# The date format used by logging formatters for %(asctime)s
# c.Application.log_datefmt = '%Y-%m-%d %H:%M:%S'

# The Logging format template
# c.Application.log_format = '[%(name)s]%(highlevel)s %(message)s'

# Set the log level by value or name.
# c.Application.log_level = 30

#------------------------------------------------------------------------------
# MyApp configuration
#------------------------------------------------------------------------------

# This is an application.

# Load this config file
# c.MyApp.config_file = ''

# Is the app running?
# c.MyApp.running = False

# Should print a warning if `MyApp.warn-typo=...` command is passed
# c.MyApp.warn_tpyo = 'yes the name is wrong on purpose'

#------------------------------------------------------------------------------
# Bar configuration
#------------------------------------------------------------------------------

# The integer b.
# c.Bar.b = 0

# Enable bar.
# c.Bar.enabled = True

#------------------------------------------------------------------------------
# Foo configuration
#------------------------------------------------------------------------------

# Some long running text: Lorem ipsum dolor sit amet, consectetur adipiscing
# elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut
# enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut
# aliquip ex ea commodo consequat.
# c.Foo.i = 0

# The integer j.
# c.Foo.j = 1

# First name.
# c.Foo.name = 'Brian'

[ankostis]

Updated the sample above to compare the new "demarcation" commit.

[minrk]

Makes sense, thanks!