[WebUI] UX improvements

[kvndrsslr]

This issue hosts a variety of rather small tasks that should not take a lot of time.

Prefix section

• [x] better dropdowns
• [x] better spacing of configured prefixes
• [x] (optionally) show url of prefix on hover

  Graph section

• [x] move import button to own branch for release
• [x] remove most context menu on right clicking operators, keep only remove and clone
• [ ] (optionally) remove minimize function in operators headers
• [x] (optionally) add keyboard handler: on backspace / del keys remove the selected operator

  Operator section

• [x] properties -> h1
• [x] node name -> h2
• [x] remove duplicate node name

  Attach files section

• [x] style button like other buttons
• [x] move button under the <hr>
• [x] (optionally) show the attached files in a nice list