Add some RAII-style wrappers for windows, children, tree nodes, etc.

[sethk]

As discussed in #2096, here is an additional header file with some RAII-style wrappers for common ImGui objects.

[learn-more]

@sethk There are some generated classes that are outdated now: PushAllowKeyboardFocus should be replaced with PushTabStop BeginChildFrame could be replaced with BeginChild(..., ImGuiChildFlags_FrameStyle, ...);, or removed outright. BeginPopupContextWindow does not take a boolean, but an ImGuiPopupFlags