embedding: begin cef string_map api

cef_string_map_alloc()
embedding: cef_string_map_size()
embedding: cef_string_map_append()
embedding: cef_string_map_find()
embedding: cef_string_map_key()
embedding: cef_string_map_value()
embedding: cef_string_map_clear()
embedding: cef_string_map_free()
embedding: rename string_list internal function to string_list instead of string_map
embedding: cef_string_utf8_cmp()
Remove some code duplication in the Compositor

It is possible to share the code which creates root layers.
Pass all key events to the current constellation frame.
Add KeyboardEvent stub.
Fill in KeyboardEvent.
Dispatch keydown, keyup, and keypress events at appropriate  times.
Implement document focus context and hook it up to click events.
Add single-line text input with no visible cursor.
Flush layout after executing timers.
More documentation.
Fix interfaces test