Labels, other UI strings should be selectable

😯 Problem to Solve

Users may want to select (for copy/paste) non-interactive text that is part of a Nimble component. Examples of this include labels (including buttons), error message text, tab names, menu items, tree items, etc.

💁 Proposed Solution

For some components (e.g. tree item), this may be as simple as removing user-select: none from the CSS.

For other components (e.g. text field), the text is a non-focusable element, and setting delegatesFocus=true (in the shadow DOM options) prevents the user from being able to select those elements. We may have good reason for setting delegatesFocus=true, so it's not as simple as turning it off.

Review the Nimble components named in the issue, including tree items, text fields, labels, error messages, tabs, and menus, and identify where selection is disabled or focus is delegated. Determine how to allow copying non-interactive text without losing the intended focus behavior. Done means the relevant UI strings can be selected across these component types while interactive behavior remains intact.