viewgroup.h: Include fix
Remove unused code
Link backtrace.cpp on Android too for consistency. Use it in FLOG.
Allow customizing the tab left/right keys.
Merge pull request #192 from unknownbrackets/keymap

Allow customizing the tab left/right keys
Port to SDL2, tested on Linux

No major changes, but it will probably require some
testing on other platforms.  Mousewheel semantics
have changed slightly and need testing.  I also
uncommented two lines to request a newer OpenGL API
on OS X.