Change how SharpDevelop handles command line arguments

[dgrunwald]

Currently, double-clicking files in explorer to open them in an existing SharpDevelop instance is rather slow, because we only forward the command-line arguments after the new process has initialized the AddIn system; we'd have to change that logic to immediately attempt this, even before showing the splash screen.

Also, we need some capability for AddIns to handle these command line arguments, and SharpDevelop should define some useful options for controlling SharpDevelop from the command line (esp. navigation commands, similar to ILSpy's command line). Such out-of-process control over SharpDevelop was requested in http://community.sharpdevelop.net/forums/t/20303.aspx