The clutter_text_get_chars() function returns a section of the
contents of the Text actor, delimited by a start and an end position.
This commit adds the implementation for that function and a test
unit that guarantees the offset-to-bytes computations are correct.
Comment why we need to enable the editability of the Text actor
inside the test suite.
This should clarify commit ea508ea528d61ae478d8bc4c88f54a89304f18e8
Whenever we are sending specially crafted KeyEvents to a ClutterText
we also need to set it editable, since the event handling code depends
on the editability setting.
Instead of changing the unit for ClutterEntry, we add a new
test unit specifically for ClutterText so that we can later tweak
it specifically for the behaviour changes needed to make ClutterText
work better.
