improve plugin architecture

[coldfix]

The plugin system was merged while still not remotely mature (to be able to continue work). I'd like the following improvements:

• retrieve plugin bindings via non-global function (makes it easier to customize bindings in madgui.App.main)
• add command line option for config file that specifies plugin bindings (for usage in an uninstalled environment)
• possibility to define order of plugin invocation (e.g. relevant for menu/toolbar hooks!)

The first two might be avoidable by pkg_resources.working_set modifications which allows to iterate entry points in a non-installed environment.

Furthermore, most events are not really fit to be entry points (possibly makes sense only for 'init'), there are basically two options:

• have non-entrypoint signal (now implemented: 9dd2502)
• add parameters (like self) if propagated via entry point

Just another thought: a very powerful ordering/dependency mechanism could work similar to the unit files in systemd.

See also: #81