Small improvements to the new Sprite Sheet dialog

[YeldhamDev]

• Added option to add all frames.
• Gave a panel styling to the ScrollContainer.
• Moved the horizontal/vertical setters to the top and removed the space between them.
• Removed unnecessary labels, as the dialog is self explanatory enough.

[reduz]

I think this could be a bit confusing. I tend to prefer linear flows on dialogs where there is only one wait to cancel and one way to confirm, but I understand the need for this. How about a button to select all frames? Something like this:

[YeldhamDev]

@reduz Made the changes requested, and also made it clear all selection when everything is already selected.

[akien-mga]

Thanks!