Feature: Enable support for modes on (almost) all Fake build settings Objects.

Feature: Enable support for modes on (almost) all Fake build settings Objects.

The only ones that are now not supposed to support this are the "modes" and "modes_default" Objects.

It should now be possible to build against clang with a command like the following:
  fake clean build -m monolithic -m clang

This is possible because of the "build_compiler-clang clang" and similar settings added to the build settings file in addition to the functionality added by this commit.