Fix label overflow fading in the wrong direction and allow custom overflow for labels

[M0rica]

Is your feature request related to a problem? Please describe.
If a label text overflows, TextOverflow.fade will be used which fades from top to bottom, which is incorrect and not very visually pleasing.

Describe the solution you'd like
Since the label text is limited to one line, simply setting softWrap = false in SidebarXCell will result in the text fading in the correct direction. Also just removing the overflow = TextOverflow.fade will allow the user to specify the overflow with a SidebarXTheme.textStyle