Introduced shared identifier metadata download component

Changes

Shared component

Only one component is needed for all sorts of identifier metadata download buttons.

Button renaming

The button is now called IDENTIFIER .XML instead of the previous name METADATA .XML to highlight that only metadata about the identifier, not the data set is downloaded.

Additionally, the identifier metadata and the subset metadata are now more visually separated, the same way it is handled with databases.

Database identifier download button

The identifier metadata download button has also been added to the database toolbar in case the database has an identifier.