Start working on adding tag columns to the library view
Add possibility to list single tags in library view (#9)
 - Update DB layer to store and retrieve a tag name for a table column
 - Add sub-menu to Tag menu item in column chooser showing all tags for the current file type
 - Implemented functionality to display tag values in columns
 - renamed some methods to differentiate singular/plural for add/delete in DB
Add possibility to list single tags in library view (#9)
 - Add/remove tag columns on show/hide all click
Add possibility to list single tags in library view (closes #9)
 - Update tag columns which can be configured when adding, updating or deleting tags (for files)
 - Remove configured tag columns when no files having this tag set are contained in the library
 - Added DBEvent type FileTagChanged to notify file tag changes throw the NotificationCenter
 - Formatting, refactoring
 - Update CHANGELOG
Merge branch 'AddTagColumns'