Restrict the usage of plugins to certain usage modes

Some plugins are only meant to be used in stand-alone mode (such as categorization). That's why the user needs to be prevented from enabling them when using the other modes.

I suggest not to hide such plugins from the plugin list if they cannot be enabled, but the checkbox in front of each should be disabled. We might consider adding a notice to the plugin description, in order to explain why the plugin cannot be enabled. The implementation might be a little tricky, because right now, plugins are enabled before a usage mode has been activated. The question is also if an plugin should stay disabled after switching from a supported usage mode to one that doesn't support it and back again.