Add one click clear button

[willerhe]

1. Added a one click clear button
2. Passed the test method
3. Updated the code from latest master branch

_{By opening a pull request, I certify that I hold the intellectual property of the code I am submitting, and I am granting the initial authors of WBO a perpetual, worldwide, non-exclusive, royalty-free, and irrevocable license to this code.}

[lovasoa]

See

• Erase all button on menu #152
• Configurable 'clear' button and canvas colour. #92

I don't think this feature is a good idea until we have a permission system

[willerhe]

Got it!