[development] New module: Module Priority

[Fabio Varesano]

> I have the code ready in my sandbox for half a year that would add before/add instead of manually setting weight once module registry lands.

For some recent development to the video module we had to explicitly
alter the weight of some modules in order to create the expected
submission flow interaction between submodules.

As example: video upload -> video thumbnailing -> video transcoding ->
video storage

All of this implemented by submodules which expect that the data
provided by the "before module" already are available.


A before/add feature would be really useful for us and I'm sure there
are lot more scenarios that would surely benefits from this.

Fabio Varesano