BUG: configure_traits should turn 'live' views to 'livemodal'.

This PR solves a problem that rears its head when using IPython in one of its GUI modes. Previously, `configure_traits()` would return immediately with no way to hold onto the root control. Eventually, a random garbage collection would delete the whole UI. Now, it creates a modal dialog that does not return control to the shell until the user has closed the dialog and thus fulfils its documented promise to block.