GUI: rename widget to ViewPort. Update GUI API usage. (#317)

applications/gui/view_port_i.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include "gui_i.h"
+#include "view_port.h"
+
+struct ViewPort {
+    Gui* gui;
+    bool is_enabled;
+
+    uint8_t width;
+    uint8_t height;
+
+    ViewPortDrawCallback draw_callback;
+    void* draw_callback_context;
+
+    ViewPortInputCallback input_callback;
+    void* input_callback_context;
+};
+
+/*
+ * Set GUI reference.
+ * To be used by GUI, called upon view_port tree insert
+ * @param gui - gui instance pointer.
+ */
+void view_port_gui_set(ViewPort* view_port, Gui* gui);
+
+/*
+ * Process draw call. Calls draw callback.
+ * To be used by GUI, called on tree redraw.
+ * @param canvas - canvas to draw at.
+ */
+void view_port_draw(ViewPort* view_port, Canvas* canvas);
+
+/*
+ * Process input. Calls input callbac
+ * To be used by GUI, called on input dispatch.
+ * @param event - pointer to input event.
+ */
+void view_port_input(ViewPort* view_port, InputEvent* event);