"Get fulltext" should check the file URL

[koppor]

We have two buttons at a file

• "Get fulltext"
  
• "Download from URL"
  

Some more "magic" should happen if the URL is pre-filled.

Regarding "Get fulltext": If the URL field is filled AND the URL is reachable AND the returned file is PDF, then this file should be used.

Regarding "Download from URL":

• A dialog opens
  
• If the clipboard contains a URL, the "URL" field should contain that URL (implemented at Download from URL now automatically fills with URL in clipboard jabref#9282)
• If the URL field contains a URL, the contents of the field should be in the "URL field".

Example:

@Article{Loesch2006,
  author    = {Felix Loesch},
  journal   = {Softwaretechnik-Trends},
  title     = {A Formal Method to Identify Variation Points in Product Line Assets},
  year      = {2006},
  number    = {2},
  volume    = {26},
  bibsource = {dblp computer science bibliography, https://dblp.org},
  biburl    = {https://dblp.org/rec/journals/stt/Loesch06.bib},
  timestamp = {Thu, 12 Mar 2020 11:24:46 +0100},
  url       = {https://pi.informatik.uni-siegen.de/stt/26_2/01_Fachgruppenberichte/04-WSR2006/24-Loesch.pdf},
}

[koppor]

Workaround: right glick on the http link - then "Download file" appears

This was not intuitive for me, because "from URL" in "Download from URL" could also mean the URL of the currently shown file line.

Idea: Put the link as initial value in the dialog opening

[koppor]

Refs #535 somehow even if the functionality is different.