High/Low DPI Adapting

The Qt UI would change the size according to the logical DPI of the computer
Only Windows and Linux have been tested. (Should also work on Mac but have not tested).
The function is unfinished.

Fixes #576 
If I comment out this line here (the mixin version), the resolution for the config window will change like to this:
![image](https://cloud.githubusercontent.com/assets/5828044/5060970/440b12de-6d30-11e4-88c0-4f85eddffd32.png)
The width is getting smaller (which is actually better, because the original version has a too large width actually).

However, if I comment out the same line in a non-mixin (previous) version. Nothing will change. (The screenshots for mixin version and non-mixin version can be found in the post above). I cannot explain how could this happen, so i cannot solve it. Is that a problem called by how the method is defined?