should be possible to place the icons at custom positions

[Mildred]

The icon view should let the user place the icons where s/he wants in the window. This is a feature that has been constantly dropped by file managers but that are very useful to visually distinguish icons from one another.

Steps to reproduce:
- open a directory with marlin
- switch to icon view (if not already in that mode)
- drag an icon in another position in the same window
- actual: the icon doesn't move
- expected: the icon should be moved where the user dropped the icon
- expected: when the user returns to the folder, the icons should be where the user last put them