DBus-activated applications

[David Faure]

kdelibs4 had KUniqueApplication, where running the app a second time would 
make a DBus call to the running instance and then quit.

The new idea, as a result of last year's freedesktop summit, is to improve 
upon this and generalize it : even the first instance can be started with a 
DBus call, using DBus activation (these .service files installed in 
share/dbus-1/services).
Which means that whoever wants to start such an application doesn't need to 
start a process anymore, but can just make a dbus call, whether the app is 
already running or not.

This makes things faster for the second invocation (no process needed), and 
allows more features: for instance we can solve the issue of leaking temporary 
files (e.g. for kmail attachments) by passing a file descriptor (to a temp 
file, unlinked by the caller) to an application via DBus, while we can't do 
that on the command line. It also allows more actions than just "open a new 
window", e.g. specific desktop files can let plasma offer "open new tab" etc.

The way to do this is fully specified in the desktop entry spec: both the DBus 
interface for doing this (called org.freedesktop.Application), and the new key 
for .desktop files to say "I can be activated via DBus".

This dbus interface is implemented by KDBusService in KF5's kdbusaddons,
which is the replacement for KUniqueApplication.
The starting of dbus-activatable apps is not implemented yet, but that's easy 
and I'll do it shortly (in KToolInvocation, I guess, and ensuring KRun ends up 
there).

What I need from you:

* if you can, make your apps "unique" by porting them to KDBusService in KF5 
(and maybe to KUniqueApplication in kde4 if you want to do things more 
incrementally).

* name the application .desktop file using reverse-dns-notation, e.g.
org.kde.kmail.desktop.
This way we can match the desktop file to the dbus service provided by the 
application, in both directions.

I (and the spec) suggest that we use org.kde.kfoo.desktop for all apps, for 
consistency, not just for dbus-activatable unique apps.
Please help renaming all the .desktop file :)

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE, in particular KDE Frameworks 5