Allow more image types for some project settings

[KoBeWi]

I noticed some project settings for images (icon, cursor) are limited to only couple of extensions, for no reason. This PR adds some additional extensions.

When splash screen is loaded, only PNG image loader is available, so that setting is unchanged.

[Mickeon]

Particularly nice for application/config/icon. Looks fine.

We should look into another way to go about this, perhaps. It can be difficult to keep track of all of these extensions if more are added.
Simpler way out, make them share the same constant String. Alternatively, the way the Image resource is imported into the project should matter, not its extension.

[AdriaandeJongh]

Does setting icons to these formats work for every export platform, or is there a conversion to a format that works for the selected platform during export? Because i'm fairly certain setting the icon of an iOS app to .bmp doesn't fly ;)

[KoBeWi]

The icon is loaded as Image before its set, so the format does not matter. Also this setting is used only for the window icon, iOS icons are configured separately in export preset.

[akien-mga]

Thanks!