A remarkable amount of people have problems telling 'plugins'
and 'extensions' apart, due to their different meaning in
different applications. So make it clearer.
We want to be able to keep extensions in a state where
they don't actually do anything but merely provide
context information. Therefore don't pass the MidoriApp.
As soon as we want to use extensions, we activate them.
Extensions are now saved in an array in MidoriApp.
