qdoc: Support use of \value command outside \enum topic.

Even though qdoc accepts the \value command(s) anywhere in the
documentation and generates tables for them, the produced output was
invalid for documentation topics other than \enum.

This change fixes the issue by not trying to resolve the enumeration
values and removing the 'Value' column for generated tables when the
\value command is used outside c++ enum documentation topic.

This enables, for example, the use of the \value command for
documenting acceptable values for QML enumeration properties,
without having to use custom lists or tables.

Task-number: QTBUG-35019
Change-Id: I597b2f9d7d03d4ab72f276752ddf53e1c405313c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
qdoc: Validate index file paths

Check for existence of the paths added by 'indexes'
documentation configuration command, and generate
warnings accordingly. Also, ensure that the strings
added with 'depends' command contain no duplicates.

Change-Id: I66a3d1b25907567bb5bfc72b78a8009d7bd8e067
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>