Set the QtWebEngine modules version to 0.9.0

Also make sure that this version number appears in the user agent
string and in the QML imports. The plan, unlike other Qt modules,
is to keep our module and QML versions in sync to make it easy
for developers to figure out what API is available in a given version.

The QTWEBENGINECORE_VERSION_STR define is explicitly set in the .pro
file since we don't call syncqt to generate a version header for the
core module.

The 0-major version currently causes the library name to start with
libQt0 instead of libQt5, but this is something that we can fix in
qtbase once we want to officially support linux desktop with
QtWebEngine 1.0.0.

Change-Id: I31915e84869b4db456416ef1f85a2296b8a06c99
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>