qdoc: Generate \keyword anchors at the top of the page

Currently, qdoc processes \keyword and \target the same way. The
negative effect of this is that when either command appears at the top
of a qdoc comment, the anchor it generates in the html file is just
above the details section on the html page. This is usually wrong,
especially for \keyword commands, where the anchor generated should
always be at the top of the page/text where the \keyword appears. This
update implements that. \target anchors are meant to be used when the
anchor should appear somewhere in the middle of the page/text, exactly
where the \target command appears.

Change-Id: Ibbbf1087562a977ff74ccd98febfe9310ecba8f0
Task-number: QTBUG-42290
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
qdoc: no longer autolinks symbols that begin with "__"

qdoc no longer tries to autolink symbols that begin
with two underscores. There may be other cases where
qdoc should not try to autolink. They should be
handled in this same place.

Change-Id: I5a39ea2bb0edec2289b8a4b1a9af1f33534093a1
Task-number: QTBUG-42441
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
qdoc: Generate the links-to-broken-links page

The cross-module link report now contains an entry
for the links to broken links subpage. It is the
last entry in the links-to-links table on the QA
page.

Change-Id: I9e0b3ba5f2efe76055902467348db878dbed9991
Task-number: QTBUG-41850
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>