general: add new files top to bottom, left to right

Until now, each new file was added to the free place nearest to the top-left corner. This means that the icons are added radially.

This patch changes this behaviour by filling the desktop top to bottom, then left to right. This is a more natural way of doing it. It also tries in more desktops than the original.

A possible enhacenment is to do it right to left when the selected language is RTL.

Fixes #95 (closed)