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.

* 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.

this should reduce the impact of the remaining 1px plot issue until we
track it down

non plugin specific class defs should not go into the plugin specific namespace