Add a priority concept to components

[tmortagne]

@vmassol

Note that the meaning of the @Priority annotation will be different for each component type. For macros for example, it means the rendering execution order when doing some XDOM rendering. How do we make that explicit when reading the code?

It’s possible we simply won’t use this annotation for macros, since it’s a bit special. It’s really something to debate on a case by case basis when someone wants to refactor it (I suspect it won’t be soon in complex cases like macros).