This patch adds support for the horizontal mouse wheel in ImGui. It
affects windows that can be scrolled, as long as the Ctrl key is not
being pressed.

The scrolling speed has been set empirically so that it matches the
scrolling speed on the Firefox browser when the horizontal wheel is
used.

Internally, it adds a MouseHorizWheel to ImGuiIO, which is then used in
NewFrame to scroll the current window.

The SDL/GL2, SDL/GL3, GLFW/GL2 and GLFW/GL3 examples has been modified
to use it.