Make editable Combobox accept substrings

This fixes the issue that onAccepted would not update the currentText or
current index if  the string was a substring/partial match.

Note that the new internal MatchFixedString case matches the complete
string in a case insensitive way.

[ChangeLog][Qt Quick Controls][Editable Combobox would not update
currentText when accepting a substring of an existing value.]

Task-number: QTBUG-35521
Change-Id: Ied850208425ae71b2414f1abf9389e9a8ed50cfb
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>