general changes
• more precise platform detection for Windows
• renamed @im_win, @unix_native
• per-method declaration of private
changes to detection of ImaeMagick installation
• all install_location and wine_unix_native things in one method
• tries better to find install location on its own (Program Files, `glob()`)
changes to temporary folder management
• imagemagick temporary folders are now in a subfolder
• `@@temp` holds now the real (operating system) temporary folder
Fix: skip shell command if array of commands is empty
• Previously it would not continue if not at least one shell command was run, waiting for a results file.
• Continue with code block if array is empty or on error
Get ImageMagick return values
• create an empty file first for the case no commands were run that create it.
• let ImageMagick output go to that file (previously that was not implemented for Windows and OS X).
• rename that output file so that file observer sees when ImageMagick is ready (previously that file would be created only then).