Import provider's icons inside the container

When creating a container, if we don't expose the provider's icons, we will end up with the default fallback for both icons and cursor theme.

This means being inconsistent with the components that are running outside the container. This may also be a usability/accessibility issue because the user defined color and/or size of the cursor will not be kept.

Fixes: https://github.com/ValveSoftware/steam-runtime/issues/311