Rename a few things with scripting.

Relates to issue https://github.com/mackron/GTGameEngine/issues/87.
Removed the old table.indexof and table.shallowcopy script functions.

These are now replaced with GT.table.indexof and GT.table.shallowcopy to make
it clearer that they are not part of standard Lua.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87
Scripting: Move GTCore.Debugging to GT.Debugging.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87.
Scripting: Move GTCore.System to GT.System.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87.
Scripting: Move GTCore.IO to GT.IO.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87.
Scripting: Remove deprecated function.
Scripting Move GTCore.Clamp to GT.math.clamp.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87.
Scripting: Move GTCore.Keys to GT.Keys.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87
Scripting: Moved GTCore.MouseButtons to GT.MouseButtons.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87.
Scripting: Move GTCore.CallbackManager to GT.CallbackManager.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87.
Remove more occurrences of "GTCore".

This commit also moves GTCore.GetDirectoryContents and GTCore.ToRelativePath
to the GT namespace.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87
Scripting: Remove the last remnants of the GTCore namespace.

This relates to issue https://github.com/mackron/GTGameEngine/issues/87.