Detect heading click and provide clicked cell in table widget

[memorycode]

Adds :selectedHeading(), which returns the table heading that was clicked this frame. It also changes :selected() to return the index of the cell that was clicked, instead of just the row, as a second return value.

IMO these methods are both misnomers because they don't do any selection on their own, but renaming :selected() would be a breaking change (and this PR aims to avoid that.)

A new table story has been added to show how these might now be used.

[oldmannt]

Thank you for creating this PR. This is exactly what I need. It looks like it's merged but not sure why I can't find those codes in the main branch.