(created a basic resizeable feature as requested. still needs tuning but works as expected at the basic level)
UPDATE #1 . 12-6-2019
I implemented a basic re-sizable feature to the old cloning tool. It still needs fine-tuning for user-friendliness but It does work as expected. I merged that into my git branch. Using "+" and "-" , you can zoom-in and out. Using "m" you can drag the image around to reach the specific part you wish to heal. I still need to add some UI bars and stuff to have both GUI and keyboard inputs but that's easy.
I found some issues in the old implementation. For example, if you do some healing and resize the canvas without explicitly saving the image using the "save" button, you healing is removed and you need to start overagain. I will investigate whether this is a bug in the ImageBrushGuideWidget only or a general problem for all tools that depend on ImageGuideWidget class.
Also note to self : For now, when I zoom-in the image a scrollbar doesn't appear. This is not a problem as anyway I can drag the image itself and this seems the more natural and userfriendly solution anyway not the scrollbar, but I will keep that in mind for later modification.
I will also experiment a little to know which key & mouse combination are more friendly for dragging, zooming, and so on.