[Feature Request] add this functions

[rahmanshaber]

i am requesting for this features.

1. I want to get the pdf document properties in a qstring so i can use it other way .so a function for getting the all the properties in qstring.
2. A function to get a page thumbnail in a Qlabel. i need this feature badly.
3. Is there a separate function to get know the file is loaded ?? if not , please implement one.

i am very much fond of your work and thanks for updating the pdf.js to v2.

[rahmanshaber]

another request
4. set the spreads and get the spreads function .so can save the spreads view.

5. set the color of background (not the pages) .if possible.
   thanks