#341 Replace "Remove Page" with "Remove Unchecked"

[James E Lang]

The "Remove Page" button can be replaced by a "Remove Unchecked"
button with no loss of functionality. This patch accomplishes that.

Any message on the History page that has not been checked will be
removed from POPFile's history if "Remove Unchecked" is selected.

[James E Lang]

[James E Lang]

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One correction to my remarks: This actually permits a skinner to choose
whether to implement Remvoe Page or to implement Remove Unchecked.