Basic D-Bus API (#866)

* feat: basic D-Bus API

Expose muted state and the number of unread message over D-Bus when
running on Linux. This is useful for, e.g., displaying notifications on
a window manager status bar.

Signed-off-by: Kristóf Marussy <kristof@marussy.com>

* docs: create docs directory

Move the documentation to a separate directory so that new documentation
can be added into one place.

We keep the following files still in the repository root by convention:

* CHANGELOG.md
* CODE_OF_CONDUCT.md
* CONTRIBUTING.md
* LICENSE.md
* README.md
* SECURITY.md

Signed-off-by: Kristóf Marussy <kristof@marussy.com>

* docs: D-Bus usage example

Signed-off-by: Kristóf Marussy <kristof@marussy.com>

* fix: remove unneeded D-Bus signals

Only notify clients that the message counts or the mute status has
changed if there actually was a change.

Signed-off-by: Kristóf Marussy <kristof@marussy.com>

* docs: rewrite sample bar client

* docs: better unread --services help

* docs: update dbus docs

* docs: use ferdium-dbus in dbus bar example

* docs: make command argument required in bar example

---------

Signed-off-by: Kristóf Marussy <kristof@marussy.com>
Co-authored-by: Victor Bonnelle <victor.bonnelle@protonmail.com>

1 files changed, 20 insertions, 0 deletions

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+# D-Bus interface
+
+Ferdium exposes an inter-process communication on Linux systems via [D-Bus](https://www.freedesktop.org/wiki/Software/dbus/).
+This allows integrating Ferdium with your desktop environment by displaying the number of unread notifications in a status area and muting or unmuting notations.
+
+## Desktop integration
+
+As an example integration, the [`docs/dbus`](dbus) folder contains a module for status bars written in Python.
+To run the example, you'll need Python 3.11 and the [`dbus-next`](https://pypi.org/project/dbus-next/) PyPI package.
+
+The integration uses the [`FerdiumClient`](dbus/ferdium_client.py) client library, which is an asynchronous wrapper over the D-Bus interface.
+It illustrates multiple advanced concepts, such as asynchronous communication with Ferdium via `asyncio` and polling the session D-Bus to see if Ferdium is running.
+
+The [`ferdium_bar.py`](dbus/ferdium_bar.py) implements a bar module to use with status bars such as waybar or polybar. See `ferdium_bar.py --help` and `ferdium_bar.py unread --help` for further indications on how to use it.
+
+## Low-level API
+
+The low-level API exposed over D-Bus is documented in [`org.ferdium.Ferdium.xml](docs/org.ferdium.Ferdium.xml) with standard D-Bus introspection syntax.
+
+Ferdium will take ownership of the bus name `org.ferdium.Ferdium` and expose and object implementing the `org.ferdium.Ferdium` interface at the object path `/org/ferdium`.