Manual screen scaling

[ceeac]

This patch adds support for manually scaling the size of the display.
This is tested and working for the SDL2 backend, but I have not tested the GDI backend.
Also, this still needs a save version bump, otherwise the setting is not saved.

[prissi]

I think there should be also a button for autodpi, as this is probably even more useful. I mean most modern application scale with Dpi as default.

I fixed the current patch for Windows, see attached.

[ceeac]

Thanks, added the button and submitted the patch in r10368.