Install.cmake: generate app bundle on mac
Engine.h: fix build
Fix include paths that use windows path separator
[mac] Fix build

clang errors out at:

std::unique_ptr<Engine> Engine(  new Engine( TITLE, WIDTH, HEIGHT, 0, 0 ) );

/Users/radical/dev/engine/src/main.cpp:25:39: Must use 'class' tag
to refer to type 'Engine' in this scope
/Users/radical/dev/engine/src/main.cpp:25:26: Class 'Engine' is
hidden by a non-type declaration of 'Engine' here

Changed that to name the variable 'engine'. IMHO, that is better anyway,
local vars shouldn't start with a capital letter.
Remove unused params from main, fix warnings