Open .omwaddon files without needing to open openmw-cs first

Tested in windows.

When I tried "open with"->"openmw-cs.exe", and tick the "use this app for all .omwaddon files", the omwaddon failed to open up.

Attempting to open the .omwaddon directly by left-clicking will only bring up the command window briefly and then nothing happens.

So right now, the only way is, to open up openmw-cs.exe first and then open your .omwaddon files.

(RM-4202 from redmine: created on 2017-11-09 by Randy Davin, )