TextEditor will now recognize an UTF-8 BOM.

A setting to determine what to do on saving has been added as well - the
options are "Always Add", "Keep Already Present" and (mostly for
completeness) "Always Delete".
Note: if encoding is "System", "Always Add" doesn't work, because there
is no API to know what the System encoding is.

Task-number: QTCREATORBUG-1857

Merge-request: 2180
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>