Adjust NavigationDrawer unset ThresholdWidth behavior

[SKProCH]

Change behavior for LeftDrawerExpandThresholdWidth/RightDrawerExpandThresholdWidth == null.

The old one:

• If set - check the width, if less make overlay mode, if more make expanded mode
• If not set - make overlay mode

The new one:

• If set - check the width, if less make overlay mode, if more make expanded mode
• If not set - make expanded mode
• (if user wants overlay mode at all time - he can just set double.MaxValue of something like that to that property)

Fix #237