Grouping of (non-fiction) books by identifier

[john-fun]

Changes:

1. .gitignore added (for bin, obj, .vs and packages folders).
2. .editorconfig modified to set the ident_size to 4 spaces (to force/match the indentation style used by the project).
3. Added Identifier column in the non-fiction books search result tab. It displays the Identifier property of the Book object.
4. Non-fiction books are grouped by the IdentifierPlain.

Issues:

1. Should the changes (3. & 4.) be applied to fiction books too?
2. Should the Identifier column be displayed or leave it hidden as before. Is it possible to show a context menu to select which columns to be visible?
3. Translations of "Identifier" column name in different languages. Using Google Translate?
4. Currently the "title" of the grouping is the group id. Anything better?
5. Color of the border of the group. Currently is hard-coded to BorderBrush="#FF002255". How to make it to follow the color theme?