WIP: Icon Grid ❤ Drag n' Drop

This merge request implements Drag n' Drop as a way to manage folders in GNOME Shell. This is currently a work in progress, although what's already implemented works reasonably well.

Tasks:

• Moving app icons from the icon grid to folders
• Moving app icons from folders to the icon grid
• Create new folder by hovering an app icon
• Add animations
  • Scale in icons moved from folders
  • Animate changed positions when moving icon to folder
  • Scale and position icon when hovering
• Visual feedback
  • When moving icon to folder (folder is highlighted)
  • When moving icon back to the icon grid (icon grid is highlighted ?)
• Custom position of icons