Update aria labels for variable fields

[maribethb]

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Problem

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Request

FieldVariable is a subclass of FieldDropdown so it will benefit from those enhancements.

The main problem is that variable names can be brief (e.g. the variable 'i') to the point of confusion when read out among all other output. This is solved by:

• Returning a qualified version of the variable as the field's ARIA value: "Variable '%1'".
• Ensuring the option for renaming a variable has a clear label: "Rename the '%1' variable" (a new message is needed here to ensure parity with DELETE_VARIABLE since the current rename message is a bit less clear than the delete variant).
• Ensuring the option for deleting a variable has a clear label: "Delete the '%1' variable" (this is the same as the current DELETE_VARIABLE message so no new version is needed).

Alternatives considered

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Additional context

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