Distinguish "adding" from "maintaining" metadata

[mih]

#319 bring the ability to use external metadata source to pull metadata into a package. This is currently implemented in the metadata widget that also hosts the git-annex metadata editor.

We likely want to start distinguishing operations that (can) manipulated metadata vs straight-up additions, and wrap them into distinct (and distincly named actions).

For example, adding CFF metadata is presently a one-way road. We end up with a file that needs to be saved, and needs to be edited by other means if needed. It would make sense to me to compile such operations into a dedicated Add metadata action group.