Fixes AppImage build

- RawTherapee app icon should be installed in hicolor theme provided in AppImage package
- Fix wrong searching path for SVG/PNG images in rtscalable.h

CMakeLists.txt

@@ -366,12 +366,14 @@ endif()
 if(NOT DEFINED ICONSDIR)
     if(UNIX)
         if(BUILD_BUNDLE)
-            set(ICONSDIR "${DATADIR}/icons")
+            if(APPLE)
+                set(ICONSDIR "${DATADIR}/icons")
+            else()
+                set(ICONSDIR "${DATADIR}/share/icons")
+            endif()
         else()
             set(ICONSDIR "${CMAKE_INSTALL_PREFIX}/share/icons")
         endif()
-    else()
-        set(ICONSDIR "${DATADIR}/icons")
     endif()
 endif()