fbd-feedback-manager: Make sure theme path components are '/' separated

Previously, assembly of the theme paths for custom paths (based on
`XDG_DATA_DIRS`) was performed using `g_strconcat`, which produced erroneous
file paths with missing path separators due to a typo in the code.

Instead, make use of `g_build_filename` to assemble the theme paths from
`XDG_DATA_DIRS` to look for custom themes, which automatically introduces
host-specific path separators between individual path components. This
introduces another temporary variable to ensure that the filename (which is
itself assembled from a name and the file ending) isn't mangled by introduction
of a path separator between these two "parts".

src/fbd-feedback-manager.c

@@ -454,8 +454,12 @@ find_themefile (void)
       // Iterate over $XDG_DATA_DIRS
       for (i = 0; i < g_strv_length (xdg_data_dirs); i++) {
         g_autofree gchar *config_path = NULL;
+        g_autofree gchar *theme_file_name = NULL;
 
-        config_path = g_strconcat (xdg_data_dirs[i], "feedbackd/themes/", comp, ".json", NULL);
+        // We leave it to g_build_filename to add/remove erroneous path separators
+        theme_file_name = g_strconcat (comp, ".json", NULL);
+        config_path = g_build_filename (xdg_data_dirs[i], "feedbackd", "themes",
+                                        theme_file_name, NULL);
         g_debug ("Searching for device specific themefile in %s", config_path);
 
         // Check if file exist