Provide DBus activation file for kmail

Testcase: now this can be used to start kmail and wait until it registers to DBus:
$ qdbus org.kde.kmail

Longer term use case: KDBusServiceStarter can be ported to DBus
activation (in the far future).

src/CMakeLists.txt

@@ -420,3 +420,7 @@ install(FILES
     ${kmail_BINARY_DIR}/src/org.kde.kmail.kmail.xml
     DESTINATION ${KDE_INSTALL_DBUSINTERFACEDIR}
     )
+
+configure_file(data/org.kde.kmail.service.in org.kde.kmail.service)
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/org.kde.kmail.service DESTINATION ${KDE_INSTALL_DBUSSERVICEDIR})
+

src/data/org.kde.kmail.service.in

@@ -0,0 +1,3 @@
+[D-BUS Service]
+Name=org.kde.kmail
+Exec=${CMAKE_INSTALL_FULL_BINDIR}/kmail