Add support for tooltips for headings.

Heading tooltips would provide explanations for what the headings mean. For the Uize.Widgets.SimpleStatsTable.Sortable, these heading tooltips would have to be integrated into the sort messaging. Perhaps this could be something that is handled by Uize.Widget.TableSort class - if it sees existing title attributes for headings, it should incorporate the existing tooltip text into the new sort message tooltip somehow.