Document all attributes to root element (#impress) in DOCUMENTATION.md.
For the main demo (/index.html) add width, height, max-scale, min-scale
attributes to the main demo. These just use the defaults, except
data-max-scale="3" which allows to scale to larger screens (HD, 4K).
For examples/classic-slides, add attributes to root element and set it
to target Full HD 1920 x 1080 resolution. Adjust CSS to match.
Add notification everywhere that defaults will change in 2021.
Changed the onclick handler to trigger the impress:console:open event
and not use the impressConsole() global function any more. The latter
is considered deprecated now that impressConsole is integrated into
impress.js itself.
Also catch some errors that appear in event handlers when the target
for the click event was immediately removed from DOM.
Fixes#651
