shell, cc-panel: Use GTYPE_TO_POINTER/GPOINTER_TO_GTYPE

The set_subpage() function already has good error checking for valid tags, so reuse that for the navigation.push action callback.

A GType is not an integer, but either size_t or uintptr_t. So casting it to int might lose bits of info, resulting in an invalid GType.

We also remove an unused struct member.

See commits for more details.

Fixes #3062 (closed)

I missed this when reviewing !2531 (merged), but that's what main is for :P