Force platform name in default user agent to WebAppManager

enyo detects platform support by analyzing the user agent string and searches for
WebAppManager to detect Open webOS. As we're not going to write our own webOS platform
integration plugin to provide a proper value for QGuiApplication::platformName we add
the "WebAppManager" string here statically.

Signed-off-by: Simon Busch <morphis@gravedo.de>