#2241 mingw-get improvements

[Hans Fr. Nordhaug]

Some missing features for mingw-get (compared to package managers on Linux):

1. List installed packages: "mingw-get list installed" (or something similar).
2. Show version number for packages: "mingw-get show <package name="">" should display version number. Or introduce new command "showdetailed".
3. Less noisy removal of packages that aren't installed. (The current output is better suitable for debugging.)