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).

Examples: Emscripten: Tweaks for size. (#2494)

Examples: Emscripten: Switched to WebGL1/ES2, Added Changelog, Updated links, Added ignore list, Fixed warning with older versions. Removed reliance on C++11 (would warn on some compiler). Improved html template, removed undesirable options, reduced log size. Tweaked main.cpp. (#2494)

Adding standalone Emscripten example.