'gtk2_ardour (msvc)' - Some changes needed before anything at all will compile

For the most part, the compilation problems are caused by lines such as:-

      WM::Proxy<ThemeManager> theme_manager;

Before MSVC will compile the above it needs to know that the specified object type contains an appropriate (i.e. publicly available) default c'tor. Therefore the corresponding header file needs to get #included, rather than just declaring that type to be a class (whilst all classes have a default c'tor, it needn't necessarily be public). The remaining changes are mostly minor.

This commit should be tested ASAP to make sure it doesn't upset the other compilers.