Provide more control for DnD features of Cinnamon and Muffin

To this point, all drag and drop threshold values have been based on the Gtk drag threshold, or a multiple of it.  This patch, along with a corresponding one in Muffin (https://github.com/linuxmint/muffin/pull/73), allows for individual control of these threshold values, as advanced settings, in the mouse and windows panel of Cinnamon Settings.