Use dbus-run-session instead of dbus-launch --exit-with-session

Summary:
dbus-launch --exit-with-session behaves badly if there's no X server running.
This resulted in it breaking when being launched from gdm or lightdm.

Also, the manpage explicitly states "This option is not recommended".

Test Plan:
Using it right now, started with sddm.
Also verified that nothing remains after logout.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11478

plasmawayland.desktop.cmake

@@ -1,6 +1,6 @@
 [Desktop Entry]
 Encoding=UTF-8
-Exec=dbus-launch --exit-with-session ${CMAKE_INSTALL_FULL_BINDIR}/startplasmacompositor
+Exec=dbus-run-session ${CMAKE_INSTALL_FULL_BINDIR}/startplasmacompositor
 TryExec=${CMAKE_INSTALL_FULL_BINDIR}/startplasmacompositor
 DesktopNames=KDE
 Name=Plasma