Version 3 discussion

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Below are my proposals for termbox2 v3. Feedback is encouraged.

1. Remove deprecated macros and functions. Notices have been in place for 2 years. This is an ABI break.
2. Add the following features. See links for rationale.
   1. Add tb_init_ex #132
   2. Add support for arbitrary escape sequences #124
   3. Add support for getting raw input #128
   4. Add support for inline TUIs #74
   5. Add a TB_INPUT_ALT_ESC input mode.
   6. Add init options for supplying a grapheme parser and grapheme width calc

I propose punting on Windows and WASM support. In order to do these properly, we either need a lot more ifdefs, a build phase, multiple source files, or some combination of those. All of those are big changes. Multiple source files with an amalgamated single-header file that we periodically or automatically rebuild remains a potential future option.