Backends: OpenGL3: Attempt to automatically detect default GL loader by using __has_include. Followup to 44cd8e3 (#2798)

Examples: SDL+DX11: Changelog, readme, batch files, fixed vcxproj, minor stylistic fixes + minor sync of other main.cpp files. (#2632)

Synced/merged minor cruft from docking branch to minimize drift. AFAIK the only meaningful no-op change is that the call UpdateHoveredWindowAndCaptureFlags() was moved above UpdateMouseMovingNewFrame() to match what docking branch has been doing for a while.

Examples: SDL: Gamepad support minor amend. Fixes ImGuiBackendFlags_HasGamepad not being set. Enable in Emscripten demo. Tweaks. (#2509, #2484).

Added support to use controllers via SDL_GameController. (#2509) ...

Updated sdl examples to use SDL_INIT_GAMECONTROLLER flag

Examples: SDL: Removed unused code. (#2484)

Examples: SDL: Using the SDL_WINDOW_ALLOW_HIGHDPI flag. (#2306, #1676) [@rasky]

Removed trailing spaces (#2038, #2299)

Examples: Setting up style before bindings, so in complex binding (vulkan/dx12) it isn't miles away from the context creation.

Examples: Misc comments mainly related to GLFW callbacks. (#1759)

Examples: OpenGL3+GLFW/SDL: Made main.cpp compile with IMGUI_IMPL_OPENGL_LOADER_CUSTOM (may be missing init). (#2178)

Examples: Using "dear imgui" terminology in all examples headers/comments + fix minor typo.

Examples: imgui_impl_opengl3 Fix GLAD error condition (#2059, #2002) ...

`gladLoadGL` returns 0 on fail, not the other way around.

Examples: OpenGL: Added support for glew and glad OpenGL loaders out of the box. (#2001, #2002). Changelog, tweaks, applied changes to SDL+OpenGL3 example.

Examples: Using fully qualified "Dear ImGui" name in window titles.

Examples: Tweaked the main.cpp example structure for all examples. (There are a few hidden agendas here: 1) I would like to avoid encouraging people from using the implicit "Debug" window, and promote using Begin/End. In spite of my best attempt, there are a few feature of the upcoming docking system that cannot work 100% properly for the implicit Debug window, so future proof let's not put that feature in the spotlight too much for new users. 2) Moved dumb hardcoded positions into a single spot that can be replaced with a viewport relative position and not affect other demo windows. 3) Calling ShowDemoWindow before anything else, also for the benefit of a specific docking demo which will have an ordering constraint which is not really problematic in a real app but shouldn't be put forward in the demo.

Examples: OpenGL3: Added shaders more versions of GLSL + made the example app default to GL 3.0 + GLSL 130 (instead of GL 3.2 + GLSL 150) unless on Mac. (#1938, #1900, #1513, #1466, etc.)

Examples: Comments, Demo: Log early out, TODO. (#1553)

Comments, documentation

Examples: Renamed imgui_impl_sdl2.cpp to imgui_impl_sdl.cpp (#1870) + changelog bits

Examples: Renamed example folders. (#1870)