month-view: make lines stronger

The current lines are very light and people
with visual issues may not find them easy.

To fix that, make these lines slightly stronger,
enough to make them clearly visible.

https://bugzilla.gnome.org/show_bug.cgi?id=758679

src/gcal-month-view.c

@@ -1533,7 +1533,7 @@ gcal_month_view_draw (GtkWidget *widget,
   gtk_style_context_add_class (context, "lines");
 
   gtk_style_context_get_color (context, state, &color);
-  cairo_set_line_width (cr, 0.2);
+  cairo_set_line_width (cr, 0.5);
   gdk_cairo_set_source_rgba (cr, &color);
   /* vertical lines, the easy ones */
   for (i = 0; i < 6; i++)