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

* 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 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.

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.