Moved plugin classes files under "src" folder.
Fix path for loading config files from plugin.
Fix test.
Fix plugin path in I18n class.
Fix plugin path in shells.
Add plugin's tests path to loader.
Make "src/" path segment part of path returned by App::pluginPath()
Add Plugin::classPath() method.

This method will return path to plugin's folder containing folders with class files.
Remove App::pluginPath() which was simply wrapper for Plugin::path().
Added 'classBase' option to Plugin::load() options.

This allows you to customize the directory under which folders containing
class files are nested.
Update plugin task to generate new default directory structure.
Merge pull request #3662 from ADmad/3.0-plugin

Sync plugin's folder structure with core.