Create new manifest which doesn't require previous version manifests for patches

The current manifests were thrown together quite quickly and there are a number of issues with them in practice.

One of the biggest problems is the fact that patch manifests require copies of each previous versions manifest to be parsed completely, resulting in lots of complex client code at the point of install to figure out what the installed layout should be.

Additionally the concept of packages which contains multiple files has proven a nuicence for patch development as it sometimes looks like it's downloading the same files many times if only some of the files in the package have changed. However, i think we should retain support for this in manifests and change the generation code to use them more sensibly.

The new style manifests should contain a complete listing of the files which make up the application. Each file should have a checksum and its unique path within the target folder.