MediaLibraryBaseModel: Use artist placeholder & update placeholders

This merge request brings changes to:

• The artist's placeholder is displayed instead of the album's one when the artist does not have any thumbnail to display.

Closes #1437 (closed)

This merge request also introduces:

• New placeholders for the albums, the artists and the tracks for both light and dark color themes.

Dark theme placeholders

Light theme placeholders

The track placeholder now fits perfectly with the iOS 16 "Now playing interface"

Now playing interface screenshots

Closes #1415 (closed)

Closes #1378 (closed)

Placeholder credits: @Corbax 😄