Add theme support by using colors from the current ThemeData

[orischwartz]

• If inBar is true, don't specify any colors -- current theme will be used automatically.
• If inBar is false, use theme.iconTheme.color for back and clear icons, theme.textTheme.display1.color for text, and theme.canvasColor for background.
• Remove colorBackButton -- color controlled by the theme.
• Don't use _clearActive to determine the color of the clear button--onPressed determines the disabled/enabled state and color/disabledColor are used.

Light theme, inBar enabled

Light theme, inBar disabled

Dark theme, inBar enabled

Dark theme, inBar disabled

[ArcticZeroo]

Thanks!