I am a web developer / designer / programmer who always has to fix your plugin every time it updates (my clients really like it). I always get the error
fatal error: php cannot re-declare class mobile detect ...
I'm not trying to be a nuisance but will your code break if you include the class outside of a function that might be called twice thus breaking php. Or perhaps (and this is what I do) change the require line from require to require_once.
The code I'm talking about is in q2w3-fixed-widget.php line 161: