[Feature/Enhancement] draw.io integration

[shlemiel]

draw.io is a web based diagram drawing tool.

I tried to integrate it briefly, but the actual implementation can vary.

fe49172

Note: To use mindforger markdown as draw.io diagram storage, noteEditor->enableSpellCheck(false); should be set since spell check greatly degrades performance of handling large texts.

• Update : I changed diagram view to fullscreen. (disabled and hided QPlaintextEdit when note type is 'Diagram')

shlemiel@69ea129