add option to show guidance hints on screen

[MartijnR]

Guidance hints are already supported, but are only shown on print outs. This proposed feature adds a Form instantiation option guidance: true or something similar that shows the guidance hint on screen (in the future we may make this collapsible but let's keep it simple now).

Requires some minor styling. For screen view a different color (a green/blue?) (no border) would probably work very well. Make sure the printout still shows guidance hint in black.

[MartijnR]

let's do <details><summary>read more...</summary>A details guidance</details>

• add to enketo-xslt
• styling in enketo-core, incl print
• remove obsolete css rules

[MartijnR]

w3c/csswg-drafts#2084

Added JS methods to uncollapse/collapse all elements