Fix DocumentTool.prototype.unmount.
Fix DocumentTool.prototype.unmount.

GerHobbelt/substance,abhaga/substance,michael/substance-1,michael/substance-1,GerHobbelt/substance,jacwright/substance,substance/substance,jacwright/substance,substance/substance,andene/substance,abhaga/substance,DuncanAForbes/substance,andene/substance,DuncanAForbes/substance,podviaznikov/substance,stencila/substance,TypesetIO/substance,podviaznikov/substance,TypesetIO/substance