Viewport: Reorganized viewport enable flags. Both user + platform + renderer need to enable a flag. (#1542)

Viewport: Give a chance to platform and renderer to actually destroy their data. Otherwise the regular ImplXXX_Shutdown + following by DestroyContext() order fails to fullifl this. (#1542)

Viewport: Replaced UpdatePlatformWindows/RenderPlatformWindows by RenderAdditionalViewports(). The update is always called in EndFrame(). (#1542)

Examples: DX12: Empty skeleton toward implementing the multi-viewport api for DirectX12. (WIP/test API) (#1542, #301)

Examples: DirectX12: Merged to new example format, imgui_impl_dx12.cpp contains the DX12 stuff, couple with imgui_impl_win32.cpp