Provide GuiStack class

This class is meant as an alternative to the existing manual screen
handling done via GfuiScreenActivate/GfuiScreenDeactivate and
GfuiScreenReplace, which is error-prone and might lead to double-free
errors.

Instead, GuiStack is a singleton class based in std::stack which
provides push/pop functions to allow switching easily between screens.

src/libs/tgfclient/guistack.cpp

@@ -0,0 +1,46 @@
+#include "guistack.h"
+#include "tgfclient.h"
+#include <stack>
+#include <cstddef>
+
+GuiStack &
+GuiStack::get()
+{
+    static GuiStack stack;
+    return stack;
+}
+
+void *
+GuiStack::top()
+{
+    GuiStack &g = get();
+
+    return g.screens.empty() ? NULL : g.screens.top();
+}
+
+void
+GuiStack::push(void *screen)
+{
+    get().screens.push(screen);
+    GfuiScreenActivate(screen);
+}
+
+void *
+GuiStack::pop()
+{
+    void *ret;
+    GuiStack &g = get();
+
+    if (g.screens.empty())
+        return NULL;
+
+    GfuiScreenRelease(g.screens.top());
+    g.screens.pop();
+
+    if (g.screens.empty())
+        return NULL;
+
+    ret = g.screens.top();
+    GfuiScreenActivate(ret);
+    return ret;
+}

src/libs/tgfclient/guistack.h

@@ -0,0 +1,20 @@
+#ifndef GUI_STACK_H
+#define GUI_STACK_H
+
+#include <stack>
+
+class GuiStack
+{
+public:
+    static void *top();
+    static void push(void *screen);
+    static void *pop();
+
+private:
+    static GuiStack &get();
+    GuiStack() {};
+    void operator=(const GuiStack &);
+    std::stack<void *> screens;
+};
+
+#endif /* GUI_STACK_H */