#74 board detect usability



Close #74

Improvements:

- autodetect falls back to manual selection if it completely fails to find a board
- autodetect only modifies the corner points if the user accepts the detection (Not pressing ESC, prevents problems when autodetect finds bogus points)
- refactored rendering text on opencv image
    - it now renders a transparent black box under the text for better readability
    - and the font size is a bit bigger
    - and it can handle new line characters properly now :)