Use D-Bus for Linux theme awareness

[J3RN]

The thrust of this change is to use D-Bus' notion of "color-scheme" instead of Dconf's notion of GTK theme, as detailed in #19.

Feature changes:

• Naturally auto-dark--is-dark-mode-dconf has become auto-dark--is-dark-mode-dbus. The new one (D-Bus) uses Emacs' built-in D-Bus integration.
• Add register and unregister functions for a D-Bus listener. (auto-dark--register-dbus-listener and auto-dark--unregister-dbus-listener). This necessitated the creation of the auto-dark--dbus-listener-object variable as a side-effect of how Emacs' D-Bus integration functions.

Other changes:

• Reduced duplication by changing auto-dark--check-and-set-dark-mode to call auto-dark--set-theme instead of duplicating the setting logic.
• Generalized the [un]registering of listeners into auto-dark--[un]register-change-listener functions that semi-intelligently dispatch to either ns-* functions for macOS or dbus-* functions for Linux.

I have some more ideas for cleaning up the macOS stuff, but I'll save those for another PR :grin:

[LionyxML]

@seagle0128 , since you were the first to implement something for Linux, I would appreciate some feedback on this take by @J3RN.

There was some discussion on Reddit and here on #19 about d-bus being a better way to check the theme state then relying on the current method that some said only worked with a specific theme.

@J3RN also, if you could provide us with some demo gif of auto-dark working under Linux with d-bus, it would be great for docs, something like what we have here: https://github.com/LionyxML/auto-dark-emacs/tree/master/images (could be another PR at another time though, your call).

[seagle0128]

Basically dbus is a more common approach, but I didn't test with many Linux distributions. I agree with the changes, but please format the codes before check-in.

[J3RN]

I rewrote the doc strings such that checkdoc no longer complains and I converted all indentation to spaces except where it was tabs before (i.e. the Applescript) as the tabs seemed rather arbitrarily placed.

This byte-compiles cleanly on my machine except for a gripe about ns-do-applescript, which is present in Emacs for Mac OS X and Emacs Plus but not in my Linux build. IMO probably nothing to worry about.

[LionyxML]

Thanks you all!

It looks like we did it. A 31th Dec merge :)

[edgar-vincent]

Thank you very much for this PR, @J3RN, it's lovely!