Issue #4202: Open .omwaddon files without needing to open openmw-cs first.
Added code to allow opening .ownaddon files from Windows Explorer when the file type is associated with openmw-cs.exe. Basically if a filename is passed as an argument to openmw-cs it will attempt to load the file directly, bypassing the startup dialogue. If no argument is passed, the startup dialogue will be displayed as it normally is.
Also tweaked the code so that if the current working directory isn't the same as where openmw-cs.exe is located, it will look for the config file in the path where the openmw-cs.exe is located. This is needed when launching with a .ownaddon file in Windows Explorer when that file is not located in the same folder as openmw-cs.exe.
First time working on something in OpenMW so not sure if someone else will need to change the status of #4202 (closed) if this is approved.