PluginManager: simplified testing for plugin load success.

Now having LoadState::Static as subset of LoadState::Loaded and removed
artificial limitation for newlib-based targets where only
LoadState::Static was present. You can now do this:

    if(manager.load("...") & LoadState::Loaded)

Instead of this (AARGH MY SANITY):

    #ifndef CORRADE_TARGET_NACL_NEWLIB
    if(manager.load("...") & (LoadState::Loaded|LoadState::Static))
    #else
    if(manager.load("...") & LoadState::Static)
    #endif
Utility: fixed Arguments documentation.