Windows build fixes.

Forgot to add exporter definition.
Utility: don't send extra newlines through NaClConsoleStreamBuffer.
Utility: verbose user-facing assert, test Unicode corner cases.
Enable GCC 4.7 compatibility for mingw32 build.

Arch's ming32-gcc is still at 4.7.
PluginManager: forgot this win-only code path while refactoring.
Utility: mingw32's GCC doesn't have std::to_string().
Utility: use nullptr instead of 0.
UseCorrade.cmake: remove old unneded cruft.

corrade_add_static() now takes one parameter less, removed dependent
functions which aren't used anywhere anymore, axed out also old and
irrelevant documentation.