Create app_suite demo

Create another example to demonstrate ways of starting an application.
Other demos covered some of these, though not the kivy language from a
string.  Each demonstration prints the source code of the executed
window onto stdout to interleave with debugging output.
Add prebuilt widget example

Add another example to the app_suite.py, showing how to use builder
during initialization to create the widget, then adding it from a
build() function.
Merge pull request #2742 from merriam/master

Create app_suite demo