Mostly const fixes in headers to make it easier for users to have more warnings enabled in static code analysis
Also updating our own rules a bit (kicking some out we won't need yet).
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6583 dfc29bdd-3216-0410-991c-e03cc46cb475
Also kicked out "Use constexpr for constants" warning in VS analyser ruleset (we haven't switched to c++11 yet)
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6402 dfc29bdd-3216-0410-991c-e03cc46cb475