Added conversions for symbols to constants, including system_icon

* Most conversions are contained in their modules. I did build a method
that could, in the future, map symbols to constants easily. It doesn't
work right now because RubyMotion doesn't include constants that aren't
present in code at compile time.
* Added a couple tests for set_tab_bar_icon

208 specifications (363 requirements), 0 failures, 0 errors