Add Qt Quick Dialogs to whatsnew in Qt 5.1

Add a Submodules section and add a link to the Qt Quick Dialogs
page.

Change-Id: Id8e18327709e1c544851f847e0bffabc713cb7e6
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Reworded a \note and removed references to ARMv5

The note was added earlier to inform the developers
that they must explicitly select one of the Qt libraries
for Android. Now that the latest installer package selects
the ARMv7 version by default, I reworded the \note.

Also, removed references to the ARMv5 builds.

Task-number: QTBUG-30173
Change-Id: I8fa764333a20faff4f3f4b8350c57b233cbd8d54
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>