Towards 2.0 release

[meadofpoetry]

So after a brief conversation it seems we are more or less agree on obus' future. Since it's a community project, and the goal is to provide a simple and robust interface to standard D-Bus IPC facilities, obus needs some simplifications.

In particular, it would be prudent to remove fancy yet unnecessary features, like IDL proxy language and PPX syntax extensions, and disentangle internal libraries and bindings.

Since these changes are quite invasive, they should go to 2.0 release. This issue is to track the progress.

• [x] Remove PPX support
• [x] Remove IDL and fix modules depending on IDL. Replace .obus files with D-Bus' native XML.
• [ ] Remove custom annotations and supplementary code
• [ ] Split obus and bindings (obus-notifications, obus-network-manager etc) into separate libraries.
• [ ] Move all the documentation to mli, making obus odoc/odig friendly

[meadofpoetry]

I would probably not have much time to deal with that in the next couple of months, but if somebody is interested in contributing, I'll be glad to help.