Composite view containers should not silently inherit the first view's title and icon

When multiple views are dragged into the same view container (e.g. Terminal + Search + Explorer side by side in the Panel), the container's title and icon are taken from whichever view happens to be first in the ordering.

This is misleading rather than merely arbitrary: the container renders as if it *were* that single view, giving no indication that other views are grouped inside it. Reordering the views silently changes the group's identity, and there is no way to tell a composite container apart from a singleton one at a glance.

### Proposal

At minimum, a composite container (2+ views) should fall back to a generic label and icon signalling "group of views", instead of impersonating its first member.

Ideally, users could rename a composite container and pick its icon — with the generic fallback as the default — the same way editor groups and profiles can be identified.

A generic default alone costs recognizability, so the rename/icon-pick affordance is what makes the fallback acceptable rather than just a different kind of wrong.

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VS Code version: 1.136.2 (88e44fa0e00b08f7758b4f6d05632e4fd5e4df6f, arm64)
OS version: Darwin arm64 (macOS 27.0)