Support "super seamless" panels

[damithc]

Current: Seamless panels give a sense of 'nested' content

Suggested: Also give a "super-seamless" (or some other suitable name) type where the content merges with the existing content more seamlessly when expanded. For inspiration, here are some examples from elsewher:

• Gmail threads showing collapsed content.
  
• GitHub showing hidden comments/code:
  
  

Justification:

• It is a way to visually convey that the content in concern is not nested but merely 'optional'.
• The feature is relevant to conent-heavy websites, which is our target market
• It is not a commonly component found (e.g., among Bootstrap components) which means it can help MarkBind stand out among competing tools

[ang-zeyu]

Hi, would like to implement this, but I have a couple of doubts with regards to the styling, mainly:

• Should the panel be non-collapsable once expanded? ( I feel this goes more with the idea of 'nested content' )
• Should a minimal header still be allowed? (e.g. "see more", "more on xxx" )

Preliminary version:

[damithc]

• Should the panel be non-collapsable once expanded? ( I feel this goes more with the idea of 'nested content' )

Not a must, but good to have. A user might want to collapse the content again to exclude it from the print (in case of user tweaking content before printing or saving as pdf)

• Should a minimal header still be allowed? (e.g. "see more", "more on xxx" )

Yes, good to have.