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

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

* 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.

* Fixed OpenEphysLib files;
* RHD2000Thread class some refactoring and stylystic fixes.

* Created 3rd slider-page on creating project steps which is used
  to select appropriate plugin, processor type, plugin GUI template,
  other options.
* Created plugin GUI templates system;
* Created some plugin GUI templates for GenericEditor;
* ButtonGroupManager and TiledButtonGroupManager classes minor
  improvements and fixes;
* Changed index of ProcessorType::UtilityProcessor;
* Other minor bugs fixes.

* Added docs for the class. Override only buttonEvent (as for other non-visual editors)
* Started fixing other editors, a lot of redundant code can be removed.
* See comments in VisualEditor.cpp

Refactoring.

* Added template files for FileSource and DataThread plugin types.
* Minor stylystic fixes and refactoring for some FileSource plugins.

* Refactoring and stylystic fixes of RecordEngine class (and one
  dependency).
* Refactoring and stylystic fixes of DataThread class and all
  appropriate dependent classes.

* Added possibility to select and generate the plugins of appropriate
  available types (Processor, RecordEngine, DataThread, FileSource).
  Now it's only generates appropriate lib files and selects some types
  in processor. However, we should generate different processor files
  for each plugin type later.
* Added possibility to select processor type;
* Some refactoring of existing code and minor fixes;
* Still needs some refactoring of newer code with some improvements.

* 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.

* Appropriate dependencies also have been changed.