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/Module.mk

@@ -0,0 +1,15 @@
+libs            += imgui
+
+src_imgui    := \
+
+cppsrc_imgui    := \
+    cimgui.cpp \
+    cimgui_impl_dx9.cpp \
+    cimgui_impl_win32.cpp \
+    imgui_demo.cpp \
+    imgui_draw.cpp \
+    imgui_impl_dx9.cpp \
+    imgui_impl_win32.cpp \
+    imgui_tables.cpp \
+    imgui_widgets.cpp \
+    imgui.cpp \