Correctly link the QML module C++ class documentation

Correctly link the QML module C++ class documentation

Previously, the links for in-depth documentation for the various C++
classes led to non-existent pages instead of the correct class
documentation.  This commit fixes the links to point to the generated
class documentation for each class.

Also, fix some broken links in the application developer guide
essentials section.

Change-Id: I2fa9e0c19f645dec696d80387c436c2288b067bf
Reviewed-by: Bea Lam <bea.lam@nokia.com>