Use System.Composition.Convention to simplify plugin development & maintainance

[biohazard999]

To support plugin developers and reduce maintainance use System.Composition.Convention for some parts of the application (where it makes sense)

• custom commands
• argument and command binding
• the pubic plugin API

relates #347

[laedit]

Thanks, you are the best!
I will review it as soon as I can.
EDIT: sorry, I am very tired, I thought that it was already the PR.

[laedit]

Just to define the perimeter more precisely, if possible I want to handle all exports through conventions, in order to really simplify all extensibility.

[biohazard999]

We can for sure get rid of most Exports but we need at least a little bit of metadata and for Imports/OnSatisfyImports I'm not sure, at least interfaces or match the PropertyTypes. But that can be tricky I guess.