Ctrl+Enter runs the query in every open tab #42

What happens

With several query tabs open, Ctrl+Enter (Cmd+Enter on macOS) runs the query in every tab instead of only
the active one.

Expected

Only the query of the active tab runs.

Steps to reproduce

1. Open two collections in tabs.
2. Press Ctrl+Enter in one of them.
3. Both tabs run their query.

Cause and fix

Each tab container rendered a view keyed by the active tab, so views multiplied and every copy listened to
the shortcut. Fixed in #47: every tab keeps exactly one view, and it only runs on Ctrl+Enter while its tab
is active.