IME problems & todo

[Soreepeong]

Problem

• Currently, IME stops working when you write something into a text input inside the main viewport, drag a window outside the main viewport, and try to input into a text input outside the main viewport.
• DispatchMessageW seems to be translating messages tangential to IME, so that its target window may change during processing, making hooking that function not usable for processing/dispatching IME messages to additional ImGui viewports, especially when we're skipping this function if the message is targeted as the main game window.
• Other methods of WndProc hooking have been done and abandoned, as using SetWindowLongPtrW or Reloaded/MinHook did not work out due to other crapwares having inadequate hook implementation.
• Hooking RegisterClassExW on Dalamud.Boot does work reliably, but it is only applicable for entrypoint load mode.

Todo

• Remove support for injection mode.
• Implement this after Rewrite ImGuiScene using TerraFX + Add DX12 backend #1542 is merged.
• Fix Korean IME dropping last character of each word, or on focus change.