Exporting split layers with group layers is broken

[novhack]

Pixelorama version: v1.0.2-stable (Steam Linux version)

OS/device including version: Arch Linux, KDE Plasma 6

Issue description: There are currently two issues I found in exporting split layers:

1. If the project has group layers these will be shown in the preview of split layer images even though they are always empty
2. When export is pressed an incorrect amount of images is created:

• For instance if there are 3 group layers and 9 pixel layers the export will create only 9 images but will include empty group layers in the export so 3 images will be missing (the export probably cycles through all layers but only exports as many as there are pixel layers)

Steps to reproduce:

1. Create a project with 3 group layers and 3 normal pixel layers in each group.
2. In export select option: Split layers
3. Empty group layers will be shown in the preview
4. Clicking Export generates 9 png files with empty images for 2 group layers and 3 last layers missing