Improve drag drop UX in file explorer and other fixes

[AdityaHegde]

• Improve drag drop UX.
  • Add a small distance before triggering the drag
  • Add icons to the dragged element.
  • Disable drag-drop for rill.yaml
• Optimise the list files APIs to not make multiple calls.

[ericpgreen2]

Drag-and-drop feels much better! (I tweaked the pixel buffer a bit)

The list files work seems to have caused the following regression:

1. Add a blank file or folder in a subdirectory. It'll be named e.g. models/untitled_file or models/untitled_folder.
2. Add another blank file or folder in the same subdirectory. It appears as if no new file/folder was created. Because the same name is being used untitled_file/untitled_folder, rather than the incremented name untitled_file_1/untitled_folder_1.

[ericpgreen2]

Approving for you to merge after fixing the above-mentioned regression.