Use KDE_INSTALL_DBUSINTERFACEDIR to install dbus interfaces

...and use PATH_VARS to make the config file work with absolute paths.

Two reasons to do this:
- DBUS_INTERFACES_INSTALL_DIR is marked deprecated
- Not hard-coding the packackage prefix is helpful on a multiarch
  layout where the prefix is /usr/${host} but arch-independent files
  should still be installed to /usr/share (i.e a level below the
  prefix).

REVIEW: 125843

CMakeLists.txt

@@ -573,7 +573,7 @@ install(
         org.kde.kwin.Compositing.xml
         org.kde.kwin.Effects.xml
     DESTINATION
-        ${DBUS_INTERFACES_INSTALL_DIR}
+        ${KDE_INSTALL_DBUSINTERFACEDIR}
 )
 install( FILES ${CMAKE_CURRENT_BINARY_DIR}/kwin_export.h DESTINATION ${INCLUDE_INSTALL_DIR} COMPONENT Devel)
 
@@ -606,6 +606,7 @@ include(ECMPackageConfigHelpers)
 set(CMAKECONFIG_INSTALL_DIR "${CMAKECONFIG_INSTALL_PREFIX}/KWinDBusInterface")
 ecm_configure_package_config_file(KWinDBusInterfaceConfig.cmake.in
     "${CMAKE_CURRENT_BINARY_DIR}/KWinDBusInterfaceConfig.cmake"
+    PATH_VARS KDE_INSTALL_DBUSINTERFACEDIR
     INSTALL_DESTINATION ${CMAKECONFIG_INSTALL_DIR})
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/KWinDBusInterfaceConfig.cmake
     DESTINATION ${CMAKECONFIG_INSTALL_DIR})

KWinDBusInterfaceConfig.cmake.in

@@ -1,5 +1,5 @@
 @PACKAGE_INIT@
 
-set(KWIN_INTERFACE "${PACKAGE_PREFIX_DIR}/@DBUS_INTERFACES_INSTALL_DIR@/org.kde.KWin.xml")
-set(KWIN_COMPOSITING_INTERFACE "${PACKAGE_PREFIX_DIR}/@DBUS_INTERFACES_INSTALL_DIR@/org.kde.kwin.Compositing.xml")
-set(KWIN_EFFECTS_INTERFACE "${PACKAGE_PREFIX_DIR}/@DBUS_INTERFACES_INSTALL_DIR@/org.kde.kwin.Effects.xml")
+set(KWIN_INTERFACE "@PACKAGE_KDE_INSTALL_DBUSINTERFACEDIR@/org.kde.KWin.xml")
+set(KWIN_COMPOSITING_INTERFACE "@PACKAGE_KDE_INSTALL_DBUSINTERFACEDIR@/org.kde.kwin.Compositing.xml")
+set(KWIN_EFFECTS_INTERFACE "@PACKAGE_KDE_INSTALL_DBUSINTERFACEDIR@/org.kde.kwin.Effects.xml")