Place new files in the location they were dropped

When files are dropped onto the desktop from an external location, the move, copy, or link happens asynchronously. By the time the GFileMonitor sees the new files and adds them to the view, the location of the drop is long forgotten. To fix this, choose positions for the new files (based on the drop location) when we receive the drop data, and store these new positions for a while (60 seconds, for now) until the GFileMonitor sees the icons created.

Closes #14 (closed)