Support for sidebar website panels

[mr-cheffy]

• [x] Display websites in mobile mode if enabled
• [x] Add context menu option to change:
  • [x] If the panel is muted
  • [x] If the user agent is mobile
  • [x] Unload the panel
• [ ] #20
• [x] #21
• [ ] #22
• [x] #19
• [x] Close and open panels
• [x] A home button to go back to the original homepage