template slots: handle slots being added after the first render

Instead of querying the slots in the shadow DOM after the first render
just add all available slots to the light DOM.

This fixes missing filled slots when slots where added after the first render,
for example if they were used only after some property is set.

The original code removed light dom templates if they were also present
in the global overrides, but it didn't remove normal slots. Let's just
keep this undefined (all slots are added now)