Fix some typos in the workbench crash course.
Merge pull request #136 from MatthieuDartiailh/workbench-course-fixes

Fix some typos in the workbench crash course.
handle null fonts in the wx backend
add an is_active property to the toolkit proxy
minor cleanup
remove need for initialization guard flags

The geometry guard is updated to be a no-op if the proxy is not active.
This removes the need to pass boolean flags to the setter methods in
order to ignore geometry updates on initialization.
set the font from within a geometry guard
cut the 0.9.2 release
Make CorePlugin.invoke_action returns the value returned by the handler.
Merge pull request #141 from MatthieuDartiailh/invoke_command_return

Make CorePlugin.invoke_action returns the value returned by the handler.
Update core_plugin.py

Update docstring for invoke_command.
remove dead code
automatically request a relayout when visibility changes

Layouts will typcially take into account the visibility of a widget.
This adds a static observer to automatically request a relayout when the
visibility of a constraint widget changes.
add a visible_widgets method to Container

This makes is simple to get the list of visible constraint widget
children of a container. The default container layout is updated to only
include visible widgets.
add configurable row and column spacing to the Form

This updates the Form layout to allow for user-defined row and column
spacing. It also fixes the layout to properly handle invisible child
widgets.
Add an example of controlling form spacing
add VGroup and HGroup convenience widgets

The are convenience Container sublasses which make it easier to create
horizontal and vertical layouts without resort to custom 'constraint'
declarations.
add a FactoryHelper constraint helper

This constraints helper allows the user to define a function (or any
callable) which will be invoked as-needed to generate the constraints
for the layout.
add an example of using a factory helper
cut the 0.9.3 release