qt: Enable ExtraMetaPanel column headers

In the "Metadata" tab of the "Media information" dialog, the columns are often poorly-sized to the contents despite calls to resizeToContents(). Having visible headers allows interactive resizing of the column widths by dragging the column separator, so that the option is always there for the user to size things to their liking.

The column headings I selected, "Property" and "Value", are admittedly somewhat arbitrary, and I'm open to suggestions for more descriptive labels. (In fact I initially went with "Tag" / "Value", then reconsidered.) In the source the corresponding variables are named simply psz_key and psz_value, so that's no help at all.