src/indicator-ng.c: Fix GtkCssProvider memory leak in indicator_ng_set_label

indicator_ng_set_label() created a new GtkCssProvider and added it to
the label's GtkStyleContext on every call, without removing old ones.
gtk_style_context_add_provider() accumulates providers, so each call
leaked a provider (~1 KB).

Since ayatana-indicator-datetime sends action-state-changed once per
second to update the clock label, this leaked ~86,400 providers/day,
causing mate-indicator-applet-complete to accumulate ~1.4 GB RSS over
7 days.

Fix by storing a single GtkCssProvider in the IndicatorNg struct and
reusing it. The provider is created on first use and cleaned up in
dispose.

Test results (100,000 iterations of set_label):
  Before fix: +99,692 KB RSS growth
  After fix:  +0 KB RSS growth

1 files changed, 8 insertions, 5 deletions

diff --git a/src/indicator-ng.c b/src/indicator-ng.c
index a94f57e..0c76df6 100644
--- a/src/indicator-ng.c
+++ b/src/indicator-ng.c
@@ -53,6 +53,7 @@ struct _IndicatorNg
 
   gint64 last_service_restart;
     GMenuModel *lMenuSections[MENU_SECTIONS];
+  GtkCssProvider *label_css_provider;
 };
 
 static void indicator_ng_initable_iface_init (GInitableIface *initable);
@@ -157,6 +158,7 @@ indicator_ng_dispose (GObject *object)
 
   indicator_ng_free_actions_and_menu (self);
 
+  g_clear_object (&self->label_css_provider);
   g_clear_object (&self->entry.label);
   g_clear_object (&self->entry.image);
   g_clear_object (&self->entry.menu);
@@ -561,13 +563,14 @@ static void indicator_ng_set_label(IndicatorNg *self, const gchar *label)
     }
 
     GtkWidget *pParent = gtk_widget_get_parent(GTK_WIDGET(self->entry.label));
-    GtkCssProvider *pCssProvider = gtk_css_provider_new();
-    GtkStyleContext *pStyleContext = gtk_widget_get_style_context(GTK_WIDGET(self->entry.label));
-    gtk_style_context_add_provider(pStyleContext, GTK_STYLE_PROVIDER(pCssProvider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+    if (self->label_css_provider == NULL) {
+        self->label_css_provider = gtk_css_provider_new();
+        GtkStyleContext *pStyleContext = gtk_widget_get_style_context(GTK_WIDGET(self->entry.label));
+        gtk_style_context_add_provider(pStyleContext, GTK_STYLE_PROVIDER(self->label_css_provider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+    }
     gchar *sCss = g_strdup_printf("label{padding-left: %ipx;}", nPadding);
-    gtk_css_provider_load_from_data(pCssProvider, sCss, -1, NULL);
+    gtk_css_provider_load_from_data(self->label_css_provider, sCss, -1, NULL);
     g_free(sCss);
-    g_object_unref(pCssProvider);
     if (GTK_IS_BOX(pParent))
     {
         gtk_box_set_spacing(GTK_BOX(pParent), nSpacing);