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/overlay/imgui/impl_spice.cpp

@@ -13,6 +13,14 @@
 #include "util/logging.h"
 #include "util/utils.h"
 
+#if !defined(IMGUI_ENABLE_WIN32_DEFAULT_IME_FUNCTIONS) || \
+    !defined(IMGUI_DISABLE_DEFAULT_ALLOCATORS) || \
+    !defined(IMGUI_USE_BGRA_PACKED_COLOR) || \
+    !defined(IMGUI_HAS_VIEWPORT) || \
+    !defined(IMGUI_HAS_DOCK)
+#error "fix imconfig.h after updating imgui version"
+#endif
+
 // state
 static HWND g_hWnd = nullptr;
 static INT64 g_Time = 0;