Now that PyInstaller respects Info.plist options, it can be used to deploy Gridsync binaries which properly launch as "agent"/background processes. Accordingly, this PR compiles the appropriate PyInstaller bootloaders on macOS at build time to enable this functionality, and creates the appropriate "Gridsync.app" bundle using PyInstaller (replacing "py2app"). I've kept the "py2app" setup options in place for now in the event that a "py2app" bundle is still needed/wanted (however, these may be removed at a future date).

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With the upstream release of PyInstaller 3.4, bootloaders should no
longer need to be recompiled in order to have Gridsync run as an "agent"
process on macOS. See also #112.