#9130 Text in main work area isn't rendered with high resolution

#9130 Text in main work area isn't rendered with high resolution
Introduce the concept of pixel ratio: the ratio of physical and device independent pixels.
This is useful for rendering of content on Retina-displays of Mac hardware with high resolution.
Qt has real support for this starting with Qt5 - therefore it has to be compiled conditionally.
This change uses some work of Marcelo Galvão Póvoa, thank you.