Add ability to set Gallery CATEGORY covers in ACP

[Meris4x4]

Hello, when creating a gallery category there is no way to set a category cover for it. It is only possible from what I discovered by creating said category. Then creating an album, uploading the cover image as a picture and set it as a category cover then HIDE the album. Way too complicated.

A picture that should be a category cover does not mean it should be a part of an album inside it.

Just like you allow to set covers for Forums - do the same for Gallery categories. Thank you.