Fix traitsui examples.

There were custom complaints that examples didn't run. This  was because
some examples don't call `configure_traits`. Also, some examples wouldn't
run on Qt; this fixes *some* of the compatibility issues.
Remove modal behavior from top-level examples.

Creating modal dialogs at the top level causes Qt windows to hang (and
doesn't really make sense). Note that the demo app creates an instance
editor with `kind='modal'` (see traitsui.extras.demo.ModalDemoButton),
so removing this from the example does not affect the demo app.
Merge pull request #96 from enthought/fix-examples

Fix traitsui examples.