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

0 files changed, 0 insertions, 0 deletions