[feat] UI option to extract metadata from multiple pages of document text

[triathematician]

Analytics:

• [x] Guess title and authors of document
• [x] Guess sections of document, with detailed text for abstracts and executive summaries
• [x] Guess references
• [x] Guess other parts of documents

User interface:

• [x] UI context menu to open metadata viewer/validator
• [x] #270
  • [x] Option to pull metadata from file
  • [x] Option to "guess" metadata from user-specified number of pages
• [x] #271
  • [x] Shows currently saved metadata (loads by default)
  • [x] Shows whether metadata is saved or not saved (with the document as represented in the viewer)
  • [x] If more than one option are available for a given metadata key, allows users to cycle through alternates
  • [x] Users can "Save" displayed metadata to update the document metadata
  • [x] Alternate icon than a save disk, since the "save" is only temporary
  • [x] Users can remove metadata from displayed list
• [x] "Save" or "Apply Changes" button in button bar, applying selected metadata updates to the document (confirm with user on save)
• [x] "Cancel" or "Close" button in button bar, to negate any user changes

Other improvements:

• [ ] Text manager view provides folder/open links to collection file (already in context menu, could be future feature)