Doc: Fixed links to Qt Multimedia's "Multimedia" overview.

-the links need the [QtMultimedia].

Change-Id: Ie251edaa88995d231248de60f62a8b527d66bba4
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Doc: Added information about WebEngine and WebChannel in "web" topic.

-increases visibility to new modules.
-also bumped the topic page to the front page.

Task-number: QTBUG-40901
Change-Id: I6e9bacbc31c19b885209f51b3485346e88fbbd8d
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Doc: Added information about WebSockets in connectivity article.

-better integration and visibility to Qt WebSockets.

Task-number: QTBUG-40901
Change-Id: I1791b2ccb686e08e12c2eaa36a4d0a92debd21bc
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Doc: Added link to Qt Creator's Qt Quick projects page to QML App page.

The link is appropriate in that section.

Task-number: QTBUG-33595
Change-Id: Ib5c2463ded9a741fe987a120d7cec40ec100351d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Doc: update Source Breaks topic

With important behavior changes from the change logs.
Fix some formatting and language issues.
Add subheadings to make it easier to find changes to
individual classes.
Where there are more than two changes to a class,
group them under a subheading.

Task-number: QTBUG-37811
Change-Id: I0b0fd56bc42bcb6f72b1c61a24865b99e1e2edad
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Merge remote-tracking branch 'origin/5.4' into dev

Change-Id: Id6d123fa2a8c594eb90f540ac5b9b86ef62aa738