Issue #3595227: scope the demo language-switcher CSS so it stops forcing admin contextual links open

Frank Mablyrequested to merge
issue/pdv-3595227:3595227-switcher-css-scope into 1.x

Follow-up bug fix for the Tugboat demo module (renamed to pdv_demo in the previous MR).

The language-switcher CSS used bare #block-pdv-demo-language-switcher ul/li/a selectors. These also matched the ul.contextual-links menu Drupal injects into the block for administrators, and an #id selector outranks core's .contextual-links { display: none } -- so the admin "Configure block / Remove block" menu was forced open over the page.

Fix: scope every rule to .language-switcher-language-url (the switcher's own list) so the contextual-links menu is left alone. Demo-only.

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