Plugin autoactivation

Discussion about plugin activation / deactivation.

= Why open this page = Some plugin must be activated by default : the first is AuthDB : https://github.com/LimeSurvey/LimeSurvey/tree/master/application/core/plugins/Authdb

But AuthDB contains all core export plugin : have a way to activate a plugin by default allow us to move the core export plugin to own plugin (core plugin) For example https://github.com/LimeSurvey/LimeSurvey/tree/master/application/core/plugins/exportDoc can really be moved to a sibgle deactivated allowed plugin.

= Solutions =

Actual solution
Add a DB upgrade to activate the plugin by default. OK for core plugin, but can not be done in other system (limeservice can have some activated plugin too for example)

Solution idea

 * Provide a config file (XML/JSON/anything ...) where we put an 'activated=>true' or something else . See for example https://github.com/LimeSurvey/LimeSurvey/blob/develop/protected/core/plugins/Authdb/limesurvey.json
 * Put a file for enabled plugin in a specific directory : https://github.com/LimeSurvey/LimeSurvey/tree/develop/protected/config/plugins for develop version : BUT (for git/gitignore) : we need to allow adding a new file in it, but allow user to remove a file .......
 * Put a file in the plugin directory : when search for plugin, if plugin is new AND have this file : activate plugin. BUT see comment https://github.com/LimeSurvey/LimeSurvey/blob/master/application/libraries/PluginManager/PluginManager.php#L202 . Directory scanning is a bad idea.