Move the user CSS settings file

Move the CSS settings file to a location based on XDG_CONFIG_HOME which
is OS dependant but with slight changes for Windows and OS X.

- Windows: CSIDL_APPDATA/Gnucash
- OS X: $HOME/Application Support/Gnucash
- Linux: $XDG_CONFIG_HOME/gnucash (or the default $HOME/.config/gnucash)

gnucash/gnome-utils/gnc-gnome-utils.c

@@ -219,11 +219,11 @@ gnc_add_css_file (void)
 #endif
     gtk_css_provider_load_from_resource (provider_fallback,  "/org/gnucash/gnucash-fallback-310.css");
 
-    var = g_get_home_dir ();
+    var = gnc_userconfig_dir ();
     if (var)
     {
         gchar *str;
-        str = g_build_filename (var, ".gtk-3.0-gnucash.css", (char *)NULL);
+        str = g_build_filename (var, "gtk-3.0.css", (char *)NULL);
         gtk_css_provider_load_from_path (provider_user, str, &error);
         g_free (str);
     }

libgnucash/core-utils/gnc-filepath-utils.cpp

@@ -543,6 +543,33 @@ get_userdata_home(void)
     return userdata_home;
 }
 
+static bfs::path
+get_userconfig_home(void)
+{
+    gchar *config_dir = NULL;
+    auto userconfig_home = bfs::path();
+
+#ifdef G_OS_WIN32
+    config_dir = win32_get_userdata_home ();
+#elif defined MAC_INTEGRATION
+    config_dir = quarz_get_userdata_home ();
+#endif
+
+    /* On Windows and Macs the data directory is used, for Linux
+       $HOME/.config is used */
+    if (config_dir)
+    {
+        userconfig_home = config_dir;
+        g_free(config_dir);
+    }
+    else
+        userconfig_home = g_get_user_config_dir();
+
+    userconfig_home = userconfig_home / PACKAGE;
+
+    return userconfig_home;
+}
+
 gboolean
 gnc_filepath_init (void)
 {
@@ -684,6 +711,24 @@ gnc_userdata_dir (void)
     return gnc_userdata_home.string().c_str();
 }
 
+/** @fn const gchar * gnc_userconfig_dir ()
+ *  @brief Return the config directory
+ *
+ *  Note that the default path depends on the platform.
+ *  - Windows: CSIDL_APPDATA/Gnucash
+ *  - OS X: $HOME/Application Support/Gnucash
+ *  - Linux: $XDG_CONFIG_HOME/Gnucash (or the default $HOME/.config/Gnucash)
+ *
+ *  @return An absolute path to the configuration directory. This string is
+ *  owned by the gnc_filepath_utils code and should not be freed by the user.
+ */
+const gchar *
+gnc_userconfig_dir (void)
+{
+    auto path_string = get_userconfig_home();
+    return g_strdup(path_string.string().c_str());
+}
+
 static const bfs::path&
 gnc_userdata_dir_as_path (void)
 {

libgnucash/core-utils/gnc-filepath-utils.h

@@ -103,6 +103,8 @@ gchar *gnc_build_data_path (const gchar *filename);
 gchar *gnc_build_report_path (const gchar *filename);
 gchar *gnc_build_stdreports_path (const gchar *filename);
 
+const gchar *gnc_userconfig_dir (void);
+
 /** Given a pixmap/pixbuf file name, find the file in the pixmap
  *  directory associated with this application.  This routine will
  *  display an error message if it can't find the file.