For Package.xml metadata subscriptions - Allow ability to resync package.xml with metadata source.

[Ben Faber]

Similar to the ‘Add to Metadata Subscription’ that can be done at any of the source folders. It would be nice to have the ability to detect files that are missing from the source directory and remove them from an existing manifest / subscription due to source branch changes or other external file system changes. I think a good verbiage for this would be “Sync Metadata Subscription with Source”

The current workaround for this is to blank the contents of the package.xml file to the top level node and then perform the ‘add to subscription’ feature at the root of the source.

[Scott Wells]

Closesly related to #943 but not quite the same.