Paint the image using QPainter.

- add a QPainter private member
- add a QPixmap private member
Paint image with QPainter
Refactor some code to prepare for multi-block display on the pixmap
Implemented horizontal stripes filling
Implemented block display
creation of README.md before pushing to GitHub
add GPL license header
Add 'Open' keyboard shortcut and remove toolbar
Update README.md
Update README.md

Update build instructions
various fixes / rework
- alignement fix
- layout rework
- view update rework
- move QLabel widget to ui file
- etc.
Restore "Open" keyboard shortcut
move #define to include file and overload resizeEvent