Implement basic preview support for G'MIC for Krita

- implement small preview window support
- implement extra feature: on-canvas preview mode (select Preview Size: On Canvas)
- a lot of small bug-fixes

Preview makes G'MIC for Krita much more usable

List of known issues is in source dir at
[calligra.git]/krita/plugins/extensions/gmic/TODO
Review it before reporting bugs, please

CCMAIL:kimageshop@kde.org