Widgets: Avoid to move focus to the Image Widget "Insert from URL" field.

Fixes a bug after [45499], where the backbone view element isn't the input field any longer. Also, managing focus programmatically is often an assumption on a specific user flow and should generally be avoided, see #43169.

Props dufresnesteven.
See #43169.
Fixes #48588.

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git-svn-id: https://core.svn.wordpress.org/trunk@46545 1a063a9b-81f0-0310-95a4-ce76da25c4cd