feat: Integrate imgui library

Version 1.91.7 of the imgui library with the cimgui wrapper.

imgui allows us to easily create minimal and powerful UI for
use-cases like in-game overlays or separate tooling.

The additional wrappers cimgui_impl_dx9 and cimgui_impl_win32
were added to provide a full C linkage integration for the rest
of the code base.

Tweaks to the makefile were kept to a minimum but enable
compilation of C++ since imgui is C++ based.

Remark: At this point bemanitools itself is still to be kept a pure
C codebase. Due to the lack of proper module/library management
with the current build system in bemanitools 5, proper separation
of concerns and clean integration with external libraries isn’t
possible with reasonable effort.

src/main/imgui/cimgui_impl_win32.cpp

@@ -0,0 +1,52 @@
+#include <windows.h>
+
+#include "imgui_impl_win32.h"
+#include "imgui_internal.h"
+#include "cimgui.h"
+
+extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
+
+CIMGUI_API bool igImplWin32_Init(void* hwnd)
+{
+    return ImGui_ImplWin32_Init(hwnd);
+}
+
+CIMGUI_API bool igImplWin32_InitForOpenGL(void* hwnd)
+{
+    return ImGui_ImplWin32_InitForOpenGL(hwnd);
+}
+
+CIMGUI_API void igImplWin32_Shutdown()
+{
+    ImGui_ImplWin32_Shutdown();
+}
+
+CIMGUI_API void igImplWin32_NewFrame()
+{
+    ImGui_ImplWin32_NewFrame();
+}
+
+CIMGUI_API LRESULT igImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
+{
+    return ImGui_ImplWin32_WndProcHandler(hWnd, msg, wParam, lParam);
+}
+
+CIMGUI_API void igImplWin32_EnableDpiAwareness()
+{
+    ImGui_ImplWin32_EnableDpiAwareness();
+}
+
+CIMGUI_API float igImplWin32_GetDpiScaleForHwnd(void* hwnd)
+{
+    return ImGui_ImplWin32_GetDpiScaleForHwnd(hwnd);
+}
+
+CIMGUI_API float igImplWin32_GetDpiScaleForMonitor(void* monitor)
+{
+    return ImGui_ImplWin32_GetDpiScaleForMonitor(monitor);
+}
+
+CIMGUI_API void igImplWin32_EnableAlphaCompositing(void* hwnd)
+{
+    ImGui_ImplWin32_EnableAlphaCompositing(hwnd);
+}