This allows us to track what state the user wanted a plugin to be in. The use

case here is, I want to disable a plugin, but the plugin can't be cleanly

unloaded. The plugin returns FALSE from its unload function with an error

stating as such.
For a GPlugin consuming application this creates a dilema where the user can't

actually disable this plugin, because as far as everyone knows, that plugin

should be loaded. With this new property, the consuming application can then

check if the desired state is set to loaded before saving it to a list of

plugins to that should be loaded at the next launch.

Updated the loader unit tests to make sure that the property is being set properly by GPluginLoader.