patch 8.2.3539: GTK3: with 'rightleft' set scrollbar may move unintentionally

Problem:    GTK3: with 'rightleft' set scrollbar may move unintentionally.
Solution:   Ignore events while moving the scrollbar thumb. (closes #8958)
diff --git a/src/gui_gtk.c b/src/gui_gtk.c
index 79a1ac3..2ea7a5e 100644
--- a/src/gui_gtk.c
+++ b/src/gui_gtk.c
@@ -1011,6 +1011,9 @@
     {
 	GtkAdjustment *adjustment;
 
+	// ignore events triggered by moving the thumb (happens in GTK 3)
+	++hold_gui_events;
+
 	adjustment = gtk_range_get_adjustment(GTK_RANGE(sb->id));
 
 	gtk_adjustment_set_lower(adjustment, 0.0);
@@ -1023,6 +1026,8 @@
 
 	g_signal_handler_block(G_OBJECT(adjustment), (gulong)sb->handler_id);
 
+	--hold_gui_events;
+
 #if !GTK_CHECK_VERSION(3,18,0)
 	gtk_adjustment_changed(adjustment);
 #endif