libsysprof-gtk: start on chart + layers

The goal here is to be able to connect data series such as
SysprofTimeSeries or SysprofXYSeries to various chart layers. Those
chart layers get stacked in a SysprofChart.

Some glue code will be necessary to go from frames into data series but
we can experiment with how to automate that later on.

src/libsysprof-gtk/sysprof-chart-layer.h

@@ -0,0 +1,48 @@
+/* sysprof-chart-layer.h
+ *
+ * Copyright 2023 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include <gtk/gtk.h>
+
+#include <sysprof-capture.h>
+
+G_BEGIN_DECLS
+
+#define SYSPROF_TYPE_CHART_LAYER (sysprof_chart_layer_get_type())
+
+SYSPROF_AVAILABLE_IN_ALL
+G_DECLARE_DERIVABLE_TYPE (SysprofChartLayer, sysprof_chart_layer, SYSPROF, CHART_LAYER, GtkWidget)
+
+struct _SysprofChartLayerClass
+{
+  GtkWidgetClass parent_class;
+
+  /*< private >*/
+  gpointer _reserved[16];
+};
+
+SYSPROF_AVAILABLE_IN_ALL
+const char *sysprof_chart_layer_get_title (SysprofChartLayer *self);
+SYSPROF_AVAILABLE_IN_ALL
+void        sysprof_chart_layer_set_title (SysprofChartLayer *self,
+                                           const char        *title);
+
+G_END_DECLS