Don't DrawMenuBar() for every menu item

Doesn't seem to be necessary to `DrawMenuBar()` on every menu item as it's added on Windows.`SetMenuTitle()` already sets a time to do that.

This seems to reduce start up time by ~3/4 second on my lenovo W510 running Win7. Also draws all core menus in menubar at once, followed by a second draw for extensions. Currently, the only extension that adds a menu it the builtin Debug menu, so this should be tested with other extensions that add menus (e.g. Emmet).

This is for https://github.com/adobe/brackets/issues/5266, but there may be more that can be done.

Windows-only.