Add onaftershow & onbeforehide events to ToolBar widget

[deblocker]

The full-screen Toolbar widget will hide or show when data-tap-toggle is enabled, but there isn't any notification event for that.

Raising events show() after Toolbar has been shown and hide() before Toolbar will be hidden can help to reposition other page elements which can profit from the full-page layout.

This will be especially useful for mobile devices (less screen space available).