Refactor: Convert C source to C++ source

- Remove dead code
- Add TODO comments on remaining X11 driver work
- Localize keybuffer
- Move fpe_handler
- Share multiply/divide between platforms
- Use externs.h for declarations
- Share finishrow between platforms
- Share lookatmouse between platforms
- Share cpu,fpu variables between platforms
- Share DivideOverflow between platforms
- Share olddacbox between platforms
- Share overflow between platforms
- Share g_dac_box between platforms
- Localize usage of tstack
- Share initasmvars between platforms
- Share calcmand.cpp between platforms
- Share fracsuba.cpp between platforms
- Share fpu087.cpp between platforms
- Organize headers in VS project
- Add PPA for gcc 4.8
- Install g++ 4.8 on travis