update to Emscripten 1.38.10; fix compiler warnings; update npm packages

change emscripten class references to interfaces

clean up using import/export/access templates

import_number_or_null for float

import/export number; import ImVec2/ImVec4 or null

pause movie if window is closed

clean up code with some macros

wrap ImGuiContext; move global variables

use opaque pointers for ImGuiContext

copy/cut/paste support in example

fix GetClipboardText

remove video url index

add images

fix up movie window

wire up Quit menu item in demo

drag and drop example from upstream

utf-8 support

track memory usage in example

AddFontFromMemoryTTF needs a native copy of the data

use optional_override instead of select_overload for lambda functions

fix indentation

reference classes return reference_ImVec2

change strcpy to strncpy

more font support; add fonts to style editor in demo

fix colors menu in demo

use raw pointer for ImGuiContext

move and rename all static globals

label reference get/set functions

remove some unused raw pointers

use references for ImGuiTextEditCallbackData and ImGuiSizeCallbackData