Move QNetworkAccessManager global instance creation to one function.

Previously the class could be created in two different places. This
adds unnecessary code duplication.
NetworkManager: Rename the class to YTNetworkManager

Pure cosmetics. Most classes in YTPlayer start to follow this naming
convention.
YTWebFontLoader: Make sure the font is not loaded twice.
NetworkOffline: Implement Retry button similar to the one in Jolla Store

Basically whenever the user clicks on the button a fake network request
to youtube.com/index.html is made. In most cases this triggers a network
selection popup. It's not 100% reliable, but it works the same in
official Jolla Store client, so it's good enough.