core: Add synced_folder plugin type
providers/virtualbox: implement the synced folder plugin
core: SyncedFolders built-in middleware
providers/virtualbox: use new SyncedFolders middleware
synced_folders/nfs: begin implementation (not functional)
core: remove NFS action
core: SyncedFolders middleware passes inoptions
providers/virtualbox: don't use NFS middleware anymore
providers/virtualbox: make sure sf host paths are strings
synced_folders/nfs:  use proper error classes
kernel: handle new NFS types
providers/virtualbox: fix broken NFS check
providers/virtualbox: output UI
core: verify explicit sf types are usable
synced_folders/nfs: config goes here
core: fix templates syntax error
Merge branch 'pluggable-synced-folder'

This enables synced folder implementations to be completely standalone
plugins and simplifies the way synced folders are enabled for providers
by providing a standard `SyncedFolder` middleware that does everything.
This is all much better tested than previously.

The built-in synced folder types are now:

* "virtualbox" - VirtualBox shared folders

* "nfs" - NFS shared folders

These can now easily be extended
Update CHANGELOG