Add DBus interface

[Lythenas]

(Partially) implements #738

Implements a basic DBus interface for PaperWM.

Functionality is pretty limited but still somewhat usable:

• methods
  • allow triggering (parameterless) actions (i.e. keybindings)
  • list actions
  • activate a space by index
  • list spaces and get single space by index
• signals
  • WindowAdded
• properties
  • DebugTrace (currently does nothing, but I'm imagining turning on some tracing functionality (e.g. more signals) in the future. Sending many dbus messages/signals does have a performance impact.

Main points of interest:

• dbus.js: implementation of the DBus interface
• dbus/org.github.PaperWM.xml: definition of the DBus interface
• dbus-send.sh and dbus-monitor.sh mainly for debugging output is not ideal to use for scripting and it can't currently call all methods

Testing Examples

• ./dbus-monitor.sh --signals and open a new window
• ./dbus-send.sh list-actions
• ./dbus-send.sh action toggle-maximize-width (or other actions)

TODOs

• write some documentation
• (probably) provide a more convenient way to use
• (probably) remove Eval and UndoEval

Open Questions

• Are we allowed to provide our own Eval? In Gnome this is already available (similarly) but only enabled if you manually set global.context.unsafe_mode = true in looking glass.
• How do we want to document this? I'm thinking just a separate md file.
• Do we want to provide some more convenient way to use the DBus interface (e.g. js or python script/library)? The output of dbus-send is not very usable is bash scripts.