Implemented QAccessibleTextWidget

Implemented QAccessibleTextWidget

A new class called QAccessibleTextWidget was added.
This class should implement all methods of QAccessibleTextInterface and
QAccessibleEditableTextInterface which only need a QTextCursor, and it
defines two pure virtual methods, to obtain and set the text cursor, so
accessible implementations of widgets which use a text cursor can implement
these two methods.

QAccessibleTextEdit is now a subclass of QAccessibleTextWidget and most of
its methods were moved to QAccessibleTextWidget.

This is a forward port of ba5d7d608cc31fc63354fd74d85a1bad7780fc45 from
Qt 4.8, and is a prerequisite for forward-porting QPlainTextEdit

Change-Id: I6093c4fa7e0a77b84de779479c6074db006efec1
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>