Change the way genres are displayed

[knjk04]

Is your feature request related to a problem? If so, please describe.
It's strange to see the genre in uppercase and for there to be underscores (as in the enum) in the new book form and the shelf grid.

Describe the solution you'd like

• Every genre to be displayed in lower case
• Spaces, not underscores, between words

For example, SCIENCE_FICTION should be Science fiction and, as a special case, NON_FICTION should be Non-fiction.

Additional context
Screenshot:

[tmrd993]

I created a pull request for this

#27

[knjk04]

Thanks!