embedding: add some commonly used string constants from chromium
embedding: implement cef_execute_process() stub

this is missing signal handler functionality in rust and we currently have no use case for actual execution of secondary processes
embedding: cef_string_wide_cmp()
embedding: cef_string_utf8_to_wide()
embedding: cef_string_wide_to_utf8()
embedding: cef_string_ascii_to_utf16()
embedding: cef_string_ascii_to_wide()
embedding: fix type decls
embedding: add cef_display_handler_t decl
embedding: add cef_life_span_handler_t decl
embedding: add cef_client_t decl
embedding: add cef_window_info_t decl
util::opts::default_opts() is now pub
embedding: redo browser creation and message loop code to be more CEF-like

big thanks to @alexcrichton for his help with this
embedding: redo browser creation, split after_created COM to separate function

this causes the callback to run at the expected time for applications that may depend on the browser loop having started or somesuch