Entry edit: Add proper warning when deleting entries

[andreoda]

Deleting entries does not show warnings and does not show possible tasks to do before or after deletion.

Problem examples:

• Song entries attached to artist and album entries will not be automatically deleted, so users should get a warning about that before / after artist and album deletion.
  Otherwise, song entries will float around the database without artists or albums attached.
• Song entries attached to album entries should not be deleted if the album is or will not get deleted later, so users should get a warning about that as well.
  Otherwise, albums will miss tracks in their track list.

As described by Pyther:

Destructive actions should ideally do the following:

• warning signal that the action is irreversible (warning colors, information text)
• require multiple clicks/confirmations
• communicate the implications of the action clearly: „this will orphan 52 song entries“ (<- we don‘t have this)

Ideas:

• Text / note (inside the deletion request box)
• pop-up
• validation warning