Added newline character between citations and other Extra field content and label for citations

[[The difference between this pull request and the other similar one is that a label is added here for the citations, see below]]

The current behavior of the plugin breaks the PMID and PMCID variables that are stored in the Extra field. In the Extra field, each new piece of data should be stored on a new line. This pull request adds a newline character between the citations and the other Extra field content. It also includes an algorithm to convert the old format to the new.

The search behavior is still potentially problematic if other data in the Extra field also includes five digits (e.g., some archive location identifiers). This pull request also adds a label (Citations: ) to the citation field to account for this. If this label is too long, it could easily be shortened to something else (e.g., GSC: ), though the general format of "XXX: " should probably be kept to be consistent with other plugins' and the core Zotero program's conventional structure in the Extra field.

The other commits regarding build numbers and builds were only there for my convenience. Please ignore and revert them.