Sprite for new itemgroup isn't displayed in the content editor until restart

[snipercup]

When making a new itemgroup and selecting a sprite, the sprite isn't shown until the game restarts.

Steps to reproduce:

• Open the contenteditor
• Create a new itemgroup
• Select a sprite for the itemgroup
• Save the itemgroup, observe the contentlist does not update
• Close and open the itemgroup. Observe that the sprite isn't loaded into the form
• Close and re-open the game. Observe the sprite is now loaded for the itemgroup