Add getText(), insert(), and remove() functions to TextEdit.

The getText() function provides access to the plain text content of a
TextEdit, and the insert() and remove() functions allow the text content
to be edited inline instead of having to assign an edited string to the
text property to make any change.

Change-Id: Id4edf0312358e2d2db45e4c649b7bd3d22f02ffa
Reviewed-by: Martin Jones <martin.jones@nokia.com>