Replace mobility bearer implementation with a shim over QtNetwork

Software built against Qt 4.6 and Qt Mobility bearer still uses the deprecated mobility bearer even when Qt on the device is 4.7 or 4.8.
There have been many bugs fixed in the bearer implementation since it was integrated into QtNetwork that are not fixed in the deprecated mobility version.
This results in low quality bug reports against QtNetwork, and difficulty in maintaining backward compatibility.

Replacing QtBearer with a shim DLL will allow binary compatiblity with applications linked against Qt 4.6 and QtBearer, while using the actively maintained QtNetwork implementation.

Anyone rebuilding their application against 4.7 or 4.8 is still recommended to migrate via the source compatibility route, as using the new APIs directly has less overhead than going via a shim.