libview: Fix some broken aspects EvViewAccessible text support

* Stop using GtkTextBuffer: It was a hack more than a real solution
* Fix setting and clearing of selection via AtkText for the current page
* Strip newline chars out of the sentence strings: Newlines break TTS prosody
* Add some logic to heuristically distinguish soft and hard returns

https://bugzilla.gnome.org/show_bug.cgi?id=725003