Refactor tabs and settings

[JoryHogeveen]

• [x] Refactor settings class into separate classes
  • Settings handler
  • Page main class
  • Tab classes
• [x] Refactor form handling, move logic into the correct class and inprove key validation
  • Keep sidebars under the sidebars key.
  • Move general settings under the general key.
  • Will also require reformat of the form class to set key prefixes.