qmldir parser: add support for "depends component version" syntax

qmldir parser: add support for "depends component version" syntax

Dependency declarations are initially for the benefit of
qmlimportscanner which does not (yet) use this parser.  This patch
adds support for dependencies into the qmldir parser for completeness,
along with autotests.  It is necessary to prevent errors at runtime
when parsing a qmldir which contains the "depends" declaration.

Task-number: QTBUG-41489
Change-Id: Ief2524a30140c42874f94f1735755b171e15dcf7
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>