Added some new hooks to PluginClassHandler to allow a VTable to specify if loaded.

PluginClassHandler::get () was designed to simply instantiate an instance of that
class for the core structure, but it did this without checking if the plugin was
loaded.

Added some new methods to PluginClassHandler exposed by LoadedPluginClassBridge
and only accessible by those who implement PluginKey to specify globally
whether or not a plugin is actually loaded, so that PluginClassHandler can
return accordingly.

Integration and unit tests added as appropriate

(LP: #1169620)
(LP: #1101026). Fixes: https://bugs.launchpad.net/bugs/1101026, https://bugs.launchpad.net/bugs/1169620.

Approved by PS Jenkins bot, Brandon Schaefer.