grid-cell: Set tooltip values using text, not markup

Tooltips are not showing up for long file names that have Pango markup characters. This is happening because the cell widget label is set using set_text, but is retrieved in the query-tooltip signal handler using get_label. While get_label can return Pango markup, it is not guaranteed to return text with markup characters escaped. Avoid this problem by fetching the file name from label and setting tooltip using text.

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