Add option to save snippet information in plain text

[delphidabbler]

Given that the code that renders snippet information when written to the clipboard already supports plain text, in addition to RTF rendering code already re-used to write RTF files.

Therefore adding the plain text option to the existing File | Save Snippets Information option and resulting dialogue box should be easier than adding other formats.

[delphidabbler]

Now pushed to develop at merge commit baaf28e