Ensure that plugin entry points are always exported on macOS and Linux (otherwise, plugins won't load in macOS release builds)

* Refactoring applied mostly because we need to satisfy the new state
  of GenericProcessor after refactoring;
* Also there were a lot of stylystic fixes applied, some minor
  improvements using C++11.

On OS X, the default directory for files explicitly saved/loaded by the user is now ~/Documents/open-ephys.  Added CoreServices::getDefaultUserSaveDirectory(), which returns ~/Documents/open-ephys on OS X and the current working directory on other platforms.

Created Xcode projects for all currently-built plugins.  Added Xcode workspace with "All" scheme for building GUI and all plugins.  Addressed some compiler warnings in plugin code.