gdk hook needs improvement

[Samueru-sama]

We've had several instances of something breaking where the issue was the gdk plugins not being added:

• fix decorations and icon issues with AppImage PancakeTAS/lsfg-vk#141

• sounddrill31/stm32cubeide-appimage@9d591f8

• fix: address gdk-pixbuf inclusion error by passing it explicitly to lib4bin pkgforge-dev/ghostty-appimage#47

Whenever libgdk_pixbuf gets added, all the plugins should be added instead.

The hook right now instead just checks if the app dlopened some of the plugins, which does not happen always.