Plugin: Fix List Conflict Files bug on posix machines

subprocess.STARTUPINFO is only available on windows machines.
The plugin now checks in before on which os it's currently running.
Plugin: Set working_dir for Show Conflict File subprocess

Use the views current folder, if no views are the plugin trys to use the
first folder which is currently opened.

If no folders are opened (why did you even run the command?) then the
default path of subprocess.check_output is used (wherever that is).