To be used for long lists that aren't interesting most of the time.

This way we only have to match triangles with their succesors.

When we matched with a rotated triangle the name of the rotated
triangle's edge was wrong.

Now Triangle can be used as the key of a QMap

This allows compilers to optimize inclusion of header files by only
running the preprocessor once per header.

This is supported by GCC and Visual Studio (as well as other compilers)
and unsupported compilers simply ignore it. However, GCC can detect the
include guards so adding the pragma will likely have no effect.