Make Dpi scaling controllable as options setting

This sugestion results from a discussion with thohartm, who recently got a 27" 4k screen, which causes Windows to scale by default to 150%.

Because of varying support of Dpi-scaling of an OS where Qt Creator runs on, there are different approaches to DPI scaling, per OS:

• MacOS: (forced) Qt Creator does not interfere and leaves it to Qt to use the system scale factor as factor for Qt Creator. -> Work mostly well!
• Windows: (opt-out) If no SCALING envoronment vaiables are set, Qt Creator instructs Qt to detect the scale factor and use it for Qt Creator -> Works best if the OS scaling is in 100% steps.
• Linux: (opt-in) Qt Creator leaves it to the user to set environment variables -> Has too many distro/windowingsystem specific pitfalls to be considered reliable.

Now, although the current behaviour still seems sane, there are cases where Windows or Linux users might want to have their opt-out/in option in a bit more comfortable way than an environment variable. Thomas' suggestion was to have a dpi scaling setting in the Options dialog. On the next run Qt Creator would read that setting (before instantiating QApplication!) and influence DPI scaling, accordingly.