ALL asset-delete buttons show "delete this sprite?" dialog

[towerofnix]

Expected Behavior

When clicking on an "X" button, the message displayed is based on what is being deleted. For example:

• "Are you sure you want to delete this sound?" when deleting a sound
• "Are you sure you want to delete this sprite?" when deleting a sprite
• "Are you sure you want to delete this costume?" when deleting a costume
• (etc for backdrops and such)

Actual Behavior

The displayed prompt's message is always "Are you sure you want to delete this sprite?".

Though these dialogs are almost certainly temporary (i.e. they will be replaced by custom popup dialogs), the cause of the issue is still worth noting.

This problem stems from all asset Selectors using the SpriteSelectorItem class. It doesn't really make sense to have costume items using the sprite selector item; there should probably be a more generic parent class (ex. SelectorItem) that SpriteSelectorItem, and potential sound/costume-backdrop classes, inherit from.

Steps to Reproduce

1. View the "sounds" or "costumes" tab.
2. Click on the "X" button inside a sound or costume's icon.
3. Read the dialog text that pops up.

Operating System and Browser

macOS 10.12.5, Chrome 58.0.3029.110