rewrite AstalRiver in vala

[kotontrion]

I rewrote the AstalRiver lib in vala. It needed some major adjustments to accommidate for the AstalWl changes anyway.

Breaking Changes

AstalRiver.River

• get_output(string name) got replaced by find_output_by_name(string name) also adds more related methods to find outputs by other means
• get_focused_output()was replaced by get_focused_output_name(). get_focused_output() now returns a AstalRiver.Output object
• run_command_async() slightly changed its signature

AstalRiver.Output

• properties associated with the wl_output were moved into the output proprty which is an AstalWl.Output
• removed the changed signal

AstalRiver.Layout

• adds the namespace property to the signal callbacks

Todo

• add doc comments
• the nix build is currently broken because it can't find AstalWl because it lives in another branch
• the set_layout_demand_callback destroy_notify parameter has scope call instead of async.

This PR depends on #371