parse options before downloading nwjs binary (since some of the nwjs binary prep depends on whether we're building a dmg)

If compiling an OSX dmg, don't try to change permission on a file which doesn't exist for that platform

Add support for loading and storing the MIDI API preference (commit 516eba76 from vanilla by aib <aibok42@gmail.com>, slightly edited.

Keep track of the number of audio and MIDI device entries in the preferences, to avoid zombie entries on Mac and Windows. Proposed fix for #213, needs testing on Mac and Windows.

another attempt to fix #218: get deterministic, sequential behavior when substituting the "nwjs" string for "Purr Data" in the OSX app bundle

Fix up misleading tooltips for the Ok/Apply/Close buttons of the Preferences dialog. These apply to the prefs dialog as a whole, not just to the audio properties.

first shot at fixing issue #218: error when trying to install on OSX. See if using a temporary text file will work around the bug of plutil creating empty files