1) Allow inserting text to move the cursor to after the inserted text
2) Implement selecting text for GtkTextView. Also assert if it's an
unsupported widget type.
3) Select an inside part of the widget, not the end.
The new test is much smaller, and the results have been carefully
looked at; GtkLabel gets it right, the others don't.
Also, temporarily include some code that dumps out useful information.
This tests a problem that was first noticed in 2003, in
bug 126797: The text of a label accessible is not properly
updated when the label switches from !use-markup to use-markup.
