showHandles method
Shows the handles by inserting them into the context's overlay.
Implementation
void showHandles() {
assert(_handles == null);
_handles = <OverlayEntry>[
OverlayEntry(builder: (BuildContext context) => _buildHandle(context, _TextSelectionHandlePosition.start)),
OverlayEntry(builder: (BuildContext context) => _buildHandle(context, _TextSelectionHandlePosition.end)),
];
Overlay.of(context, debugRequiredFor: debugRequiredFor).insertAll(_handles);
_handleController.forward(from: 0.0);
}