imgui: reorganize imgui library files to mirror what's in the imgui releases (#285)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Functionally, no change.

Move ImGui library files around so that they mirror how they are
structured in imgui releases. This will make it easier to migrate to a
future version of ImGui.

Add a handful of static asserts to check that we have the necessary
features enabled / disabled in imgui header.

## Compiling
👍 

## Testing
Tested 32-bit overlay, 64-bit overlay, TDJ, and UFC overlays, and
spicecfg.

Interesting finds during ImGui testing (not part of this PR, but for
future reference when upgrading imgui version):
* Updating dx9 backend to the newest causes TDJ to stop rendering the
overlay completely (fine in UFC or any other game)
* Omitting `IMGUI_USE_BGRA_PACKED_COLOR` causes spicecfg software
renderer to use the wrong color (RGB format parsing error probably)
* At some point - v1.90? the `About` and `Licenses` buttons in spicecfg
stop responding after an upgrade, maybe our improper usage of MenuItem?
* Using the docking branch of ImGui is not required but we should just
stick to it for multi-viewport functionality which we may want to use
later.

src/spice2x/CMakeLists.txt

@@ -211,7 +211,7 @@ add_compile_definitions(
 #set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DACIOEMU_LOG")
 
 # add project directory to include path so we can comfortably import
-include_directories(${spicetools_SOURCE_DIR})
+include_directories(${spicetools_SOURCE_DIR} ${spicetools_SOURCE_DIR}/external/imgui)
 
 # add external libraries
 add_subdirectory(external/fmt EXCLUDE_FROM_ALL)
@@ -516,7 +516,6 @@ set(SOURCE_FILES ${SOURCE_FILES}
         overlay/overlay.cpp
         overlay/window.cpp
         overlay/imgui/extensions.cpp
-        overlay/imgui/impl_dx9.cpp
         overlay/imgui/impl_spice.cpp
         overlay/imgui/impl_sw.cpp
         overlay/windows/acio_status_buffers.cpp