Use local paths in public headers

Angle-bracket paths require inclusion of Insanity header paths in
commandline (or project settings for GUI editors).
Correct in-class initialization

MSVC didn't correctly support in-class initializer-list initialization,
and now errors on it. However, it allows the correct syntax, so just do
that instead.
Improve Mod loading code

Handle error conditions, such as no such mod, no exported initializer,
and initializer returning null. Return nullptr in all such cases, and
let application deal with it.
Remove obsolete comment