Add context menu for node actions #381

Original idea: #345 (comment)

Let's implement a context menu against each node in rich editor.
Context menu will contain all options related to selected node where user opens the context menu.

For example, when user open context menu on link there will be options to

• Copy URL
• Open external link (in case it is external link)
• Open note (in case it is link into another note)
• Download file (in case it is a resource link)
• Edit URL
• Unlink

For image it will be

• Download image
• Open full image
• Edit (to edit URL and alt text)
• Remove

There will also be general actions like

• Add link... (for selected text only)
• Add image...
• Upload file...
• Insert current date
• Insert page break
• Formatting (for selected text and inline nodes)
  • Bold
  • Italic
  • Strike-through
  • Code
  • Clear formatting
• Block type
  • Paragraph
  • Quote
  • Code block
• List type
  • Unordered list
  • Ordered list
  • Task list
• Header (with sublist for 6 variants)
• Remove (for a block nodes)

Details

• Each option can have nested options if necessary, but options list must be designed to run quick and frequent actions instantly with no frictions
• Option may show user shortcuts to do the action even quicker, with no context menu opening
• We must test options in browser tests. We can mock context menu and check requested menu structure, and optionally call the callbacks like user would do.