on-tab-change (augmentable only) in drscheme:unit:frame<%>

Called after a new tab becomes the selected tab in the frame.

• (send a-drscheme:unit:frame on-tab-change from-tab to-tab) -> void
     from-tab : (is-a?/c drscheme:unit:tab<%>)
     to-tab : (is-a?/c drscheme:unit:tab<%>)

  The from-tab argument is the previously selected tab, and the to-tab argument is the newly selected tab.