Remove unused variable
Support running multiple instances on OS X (fix #533)
Don't initialize GuiSystem if it's not necessary
Minor check in ~ScopedHandle()
Don't load a file if it's already open on drop files event

This avoids to open a document two times on Mac OS X
from the command line (the file is loaded when we
process the command line, and by Finder, which send us
an application:openFile: message).
Update version to 1.0.8-dev