Added Xcode project for OpenEphysHDF5Lib, which builds a shared library (libOpenEphysHDF5.dylib) and installs it in open-ephys.app/Contents/Frameworks.  On macOS, the KWIKFormat plugin now links against libOpenEphysHDF5.dylib.

Added Xcode config files for building dylib's that are embedded in open-ephys.app/Contents/Frameworks

Symbol visibility fixes

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

Mac build updates

Fix Julia path and flags in Makefile

The Makefile of the JuliaProcessor plugin used
absolute file paths, which prevented compilation.
I have replaced the absolute path with a shell
macro as suggested by Julia's embedding
documentation.

Fixing this revealed a second problem: the linker
flags were incorrect. I have replaced the
offedning flag '-ljulia' with build flags
generated by julia, as per Julia's embedding docs.

See issue #80

Added Plugin Generator. Updated to JUCE4.

* Added possibility to auto-save/load parameters for each created
  plugin.
* Some fixes needed for LookAndFeel code generation.
* Fixed plugin type wasn't loaded correctly into PluginGenerator after
  restarting.
* Fixed boolean parameters stored incorrect values inside.
* Fixed some bugs with parameters.

* Changed the order of plugin types at the enum definition.
* Changed some plugin definition names to be more user friendly and to
  be consistent with OE GUI.
* Removed "DataThread" plugin type from the "Plugin type" comboBox at
  the plugin creation page. Now to create DataThread plugin user
  should select "Processor" plugin type, then "Source" processor type
  and click "Use separate thread" button
* Removed possibility to create "Utility" processors plugins since we
  don't want to let users create such types of plugins.
* Removed possibility to generate editor and content component files
  for non-processor plugins since this feature was redundant.
* Removed "Plugin" page from "Config" for non-processor plugins since
  they have no built-in parameters.
* Added missing LookAndFeel related files to stage.
* Refactoring.