Allow the NVDA desktop shortcut (and therefore ctrl+alt+n) to restart NVDA if its...

Allow  the NVDA desktop shortcut (and therefore ctrl+alt+n) to restart NVDA if its already running, even on Windows XP. To do this a new argument  has been added to nvda_slave, called launchNVDA, which executes NVDA with any other arguments given, and does not wait for NVDA's process to exit. The desktop shortcut now executes nvda_slave.exe launchNVDA -r. Fixes #886