Refactor window-system configuration
This change streamlines the window system selection code in configure.in and moves many common function declarations from window-specific headers to frame.h. It introduces a new TERM_HEADER macro in config.h: we set this macro to the right header to use for the window system for which we're compiling Emacs and have source files include it indirectly. This way, we don't have to teach every file about every window system.
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