Let MonoDevelop 4.0 re-arrange the .sln and .csproj files

This is just the result of opening the solution properties without
changing anything, to let MonoDevelop 4.0 re-arrange the .sln and
.csproj files as it wants. This should prevent noise in later commits
that contain real changes.
Switch csproj files to Mono/.NET 4.0 target framework

This also upgrades the solution file to the Visual Studio 2010 format,
and updates Hyena to the latest git master, that now also targets the
4.0 framework.

This only impacts the build when using xbuild or building from the IDE.
The usual 'make' build is not affected.
Banshee.sln: Adjust naming convention policy to match our guidelines

Only real adjustment is to have lower case parameter names, the rest is
default values as set by MonoDevelop. Further refinements might be
needed to match our guidelines as described in the HACKING file, which
is still the reference.
build: Fix various issues when building with xbuild

Add some missing assembly references, in particular gio-sharp.

There are some issues left, but they seem mostly limited to projects
with platform-specific dependencies: OSX and Ubuntu-specific extensions.

Also update Hyena to the latest git master, to bring in some fixes in
its csproj files.