Fix bug in finding icons for suffixed package

The issue was in determining the icon file for the .desktop file. It
had assumed that the icon already existed. This assumes that the icon
file exists only with the "nightly" and "unstable" suffixes.

CMakeLists.txt

@@ -296,10 +296,10 @@ if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
   set(${output_var} ${_first}${_rest} PARENT_SCOPE)
   endfunction()
 
-  function(install_desktop_files filename app_name exec_cmd icon_file)
+  function(install_desktop_files filename app_name exec_cmd icon_file icon_install_file)
     set(DESKTOP_NAME ${app_name})
     set(DESKTOP_EXEC_CMD ${exec_cmd})
-    get_filename_component(DESKTOP_ICON ${icon_file} NAME_WE)
+    get_filename_component(DESKTOP_ICON ${icon_install_file} NAME_WE)
     set(_output_file ${CMAKE_CURRENT_BINARY_DIR}/${filename}.install)
     configure_file(
       ${CMAKE_CURRENT_SOURCE_DIR}/installers/LinuxInstaller/launcher-template.desktop.in
@@ -309,34 +309,44 @@ if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
             RENAME ${filename})
     install(FILES ${icon_file}
             DESTINATION /usr/share/pixmaps
-            RENAME ${DESKTOP_ICON}.png)
+            RENAME ${icon_install_file})
   endfunction()
 
+  set(_icon_suffix)
+  if(CPACK_PACKAGE_SUFFIX)
+    if (${CPACK_PACKAGE_SUFFIX} STREQUAL "nightly" OR
+        ${CPACK_PACKAGE_SUFFIX} STREQUAL "unstable")
+      set(_icon_suffix ${CPACK_PACKAGE_SUFFIX})
+    endif()
+  endif()
+
   set( IMAGES_DIR ${CMAKE_CURRENT_SOURCE_DIR}/images )
   set(_app_name_suffix)
   if(CPACK_PACKAGE_SUFFIX)
     uppercase_first_letter(_app_name_suffix ${CPACK_PACKAGE_SUFFIX})
   endif()
   if(ENABLE_MANTIDPLOT)
     set(_icon_filename ${IMAGES_DIR}/mantidplot)
-    if(CPACK_PACKAGE_SUFFIX)
-      set(_icon_filename ${_icon_filename}${CPACK_PACKAGE_SUFFIX})
+    if(_icon_suffix)
+      set(_icon_filename ${_icon_filename}${_icon_suffix})
     endif()
     install_desktop_files(mantidplot${CPACK_PACKAGE_SUFFIX}.desktop
                           MantidPlot${_app_name_suffix}
                           ${CMAKE_INSTALL_PREFIX}/bin/MantidPlot
-                          ${_icon_filename}.png)
+                          ${_icon_filename}.png
+                          mantidplot${CPACK_PACKAGE_SUFFIX}.png )
   endif()
 
   if(ENABLE_WORKBENCH)
     set(_icon_filename ${IMAGES_DIR}/mantid_workbench)
-    if(CPACK_PACKAGE_SUFFIX)
-      set(_icon_filename ${_icon_filename}${CPACK_PACKAGE_SUFFIX})
+    if(_icon_suffix)
+      set(_icon_filename ${_icon_filename}${_icon_suffix})
     endif()
     install_desktop_files(mantidworkbench${CPACK_PACKAGE_SUFFIX}.desktop
                           MantidWorkbench${_app_name_suffix}
                           ${CMAKE_INSTALL_PREFIX}/bin/mantidworkbench
-                          ${_icon_filename}.png)
+                          ${_icon_filename}.png
+                          mantid_workbench${CPACK_PACKAGE_SUFFIX}.png)
   endif()
 endif()